Why isn't something done about this?

Cleaning staff should remove the needles. Security or police arrest the dealers, and remove people loitering on stations or trains.

Is it a lack of funding, laws, sympathy for the homeless, or ... what?

I lived in London for ten years, and homeless people, or beggars, were extremely rare on public transport vehicles, and even rarer within stations. I don't know what combination of law and enforcement is used, and there are homeless people on the streets, but something prevents them from overtaking the transport system.

I'll offer up this explanation from arch-conservative James Burnham (so please don't downvote the messenger) just for consideration on the idea that the "doing something about this", as SF tries so mightily, can make the problem worse. This is from 1964:

But what exactly is Skid Row? In reality it is not, other than incidentally, a spatial concept, but a functional one. . . Skid Row is the end of the line, and there must be an end of the line somewhere. ... The whole [reform] operation has proved to be, inevitably, an ideological illusion. Since Skid Row is not a static place, it cannot be abolished or rubbed out. The most noticeable consequence of this anti-Skid Row campaign has been merely to diffuse Skid Row, for a while, throughout the City.

I've elided a lot of supporting argument, the gist of which is that the denizens of Skid Row tend to form networks of support & attract institutions that maintain them. And while that level of maintenance looks (and is) rather squalid and miserable to outsiders, breaking it up is just replacing an organic support network with an artificial one which looks at the symptoms more than the roots.

I can't speak to London except that I guarantee London has a Skid Row, even if it's not physically located in London, if that makes sense.

Final disclaimer that I don't claim to have the answer, just pointing out one point of view that seems rather pertinent to 2017 SF despite being written 53 years prior, whatever moral valence you assign to the author & his politics.

I'm not suggesting solving the whole problem. London hasn't -- see, for example, the events that led to the fire and many deaths in Grenfell Tower last week. I'm out of touch, but I think that building was one step above Skid Row.

What I am suggesting is removing the homeless people from the public transportation network, so it's kept cleaner and feels safer for the users.

I think the difference is that the US has abject poverty at levels that call into question its status as a proper first world country. So many of these people probably simply require some basic mental health/rehab services, but they just have too many hoops to jump through to get them (assuming they qualify for Medicaid or other assistance, which isn't necessarily a given). It's not surprising that a country like Britain with reasonably accessible social services has way fewer people in this condition. To your question about why they've taken over public transit in particular - I think the situation is just that out of hand. Libraries and parks are similarly full of homeless people, probably just due to the fact that they're public.

SF attitudes towards certain behaviors make hard to do what most cities do, which is arrest these people and make them go somewhere out of sight.

There are cleaning staff going through the area cleaning stuff up. But people still are doing their crap and the laws are unenforced because of those political reasons.

I also think SF police & the city decided the tenderloin was the homeless containment zone long time ago with SROs being there. That is also where the civic center station is.

I'm sure they clean the Civic Center station area daily. If they didn't, it would be knee deep in filth.

That's just where they've decided to gather. I'm not entirely sure what attracts them there, other than the burger king with the huge "EBT Cards Accepted" poster.

all of the above; plus a janitor or a police officer cost $200k+/year each when you factor in pay, benefits, overtime. So you can imagine they might be a little thinly staffed.

http://www.mercurynews.com/2016/11/01/bart-janitor-grossed-2...

The UK has a population of 60 million people. There are only about 4500 rough sleepers.

This figure has risen every year for the past 6 years, and it's likely to keep rising. Removing the single room supplement / imposing the bedroom tax, freezing housing benefit, cuts to local authorities (and thus cuts to drug and alcohol services, and to some mental health provision, and to some housing provision) all mean that rough sleeping is going to increase.

And that number doesn't include people who are in temporary accommodation or vulnerably housed or sofa surfing.

Every complaint I've seen on this topic is about how repulsive or unsafe it feels for someone commuting to their job in SF. No one talks about these thousands of people who are suffering from drug addiction, mental disorders, socioeconomic injustice, and all kinds of abuse. No one's complaint is: "how can we, as society, allow these conditions to persist, why aren't we helping these people in a meaningful way?" It's the apathy of the population in general that allows this kind of situation to come about and then persist despite the best efforts of "they" who are supposed to "do something about it."

Millions of homeless people in hundreds of other cities around the world suffer from the exact same conditions you've described. Yet nowhere but the Bay Area have I seen those conditions caused those people to behave the way they do in SF. The city has spent hundreds of millions of dollars on safe injection sites, needle disposals, and public restrooms, and yet still must continue to spend money to pay city employees to clean up needles and human waste from the streets and transit stations.

Homelessness is a massive problem in SF, but the attitude that they behave this way "because of the situation they're in" is untrue and counterproductive.

I disagree with that strongly. There are loads of people describing what they see of the homeless issue, and proposing or discussing potential solutions in a generally respectful way. For the most part, the evidence is described more as remarkable than repulsive. Whenever the topic comes up on HN, I think the discussion is very reasonable and considerate, and I always find it an interesting read.