I'll just add by the way (since apparently I'm being downvoted for speaking a snarky comment), that fare enforcement seems almost non-existent in both SF and Paris. Something about it being too expensive or dangerous to actually make people buy tickets. But policing your rules is an important thing for having people believe in the system, as unpopular as it may be. And maybe how safe/trustworthy the system is. Which neither SF nor Paris are shining examples of.
I don’t think danger was a problem in Paris. They always work as teams of ~5 people and I’ve never heard of them being specifically assaulted (unlike, say, bus drivers, unfortunately).
In any case, London seem much better, both in terms of (perceived) safety and fare jumping. On the other hand, there are TfL agents everywhere (which is expensive, but you can’t jump the gates), and the fares are much higher.
If there are rules, enforce them. Don't make the law abiding pay and look the other way for the ones breaking the rules.
Why, indeed? This is a logical conclusion.
I think you can quibble about means testing, barriers to application, and just “do it through taxes”. Amusingly, we’re quite similar to Paris in that the SFMTA “farebox recovery ratio” (fare revenue divided by costs) is in the 25% range [2].
That low funding rate (75% through taxes then) is despite fairly large increases in the fare over time. IIRC, the youth / senior Pass was $10 in 2007 and is now $40, the regular was $50 and is now... $81 (and set to increase in January [3]). Somewhat rationally, it seems this was all connected to indexing the fares to Bay Area inflation [4], designed so that salary of the drivers keeps up with inflation.
Like Paris though, we have (had) a large tourist contingent that pays full fare when visiting. Some argue that “they’re not taxpayers, make them buy tickets”, but it seems more efficient to just account for that in hotel taxes. Otherwise you never get out of the fare collection inefficiency (in San Francisco, drivers on buses must also be fare enforcers, historically leading to it being a dangerous job) and again it penalizes people who don’t know to apply for their free passes they’re entitled to.
[1] https://www.sfmta.com/fares/free-muni-youth
[2] https://www.sfmta.com/reports/muni-farebox-recovery-ratio
[3] https://www.google.com/amp/s/sf.curbed.com/platform/amp/2020...
[4] https://sf.curbed.com/2019/6/19/18691884/muni-price-inflatio...