They require water, which requires irrigation (may not already be in place and of course has logistical problems), trees also root systems can lift and crack sidewalks/pipes/other things, you now have biological contaminants, there's bugs and fungus and things that can spread, and so much more.

I'm not saying we shouldn't do this. I really like seeing plant life in cities and I think it really makes a psychological difference (I _love_ living in a city with a lot of green spaces and trees everywhere). But it is nowhere near as simple as placing a tree in the ground and cleaning the leaves.

"now have biological contaminants, there's bugs and fungus"

Dude, we have rats running around, pigeons nesting and pooping, what biological contaminants?

Just because biological contaminants exist doesn't mean adding more creates more issues. More trees also means more rats and pigeons. What's your argument? That just because we already have a bad thing doesn't mean that a similar bad thing isn't bad? That doesn't make much sense and is a pretty lazy argument.

Either way, rats and pigeons feed on garbage, they dont eat oak trees.

We need to have less takeaway left on the street and garbage bins that are cleaned up on time.

How much irrigation that is needed very much depends on where you are and what kind of tree. After the first few years it take to settle, trees have deep roots and rarely need watering.

And I have never heard of trees contribution negatively to "biological contaminants", whatever that means. Rather, trees contribute to a cleaner and healthier environment.

Tree limbs die all the time. Normally when disease or damage kill a tree branch, it isn't a big deal because the tree is in a forest and probably won't fall on anybody or damage any property under it. In a city, you can't ignore a widowmaker. The risk of it damaging property or flat out killing somebody is too high, you have to pay to get it safely removed. Often this becomes "end-of-life removal" because it's deemed safer or cheaper in the long run to remove the tree entirely. So now you have a green mostly alive tree being chopped down because one of the limbs died, and everybody in the neighborhood thinks you're a tree butcher but nobody else wants to chip in to have it maintained.