The climate is nice. They can survive the winter without having to crawl into a steamy sewer like rats. Though many cats sleep under cars (especially if the engine is still warm). Most cities in these areas are not heavy on traffic, the city roads are narrow and designated for slow-driving - giving the animals an opportunity to move around with not much noise and danger.
Old people in the neighborhood feed them and play with them. I would say it's as nice for them as it is for the cats (they get to have a hobby, go out etc). People at cafes and restaurants (especially the ones with outdoor seating) might put some food aside.
People growing up in such an environment learn to like the cats, and give them some attention or feed them themselves. (I grew up loving stray cats, and animals in general due this culture). Was surprised to see how much having a pet is a "business" in the states (higher rents if you have a pet, mandatory chips, pet salons everywhere, eugenic like focus on perfect looking breeds etc).
And the elephant in the room. Careless people adopt them, decide it's too much work and then let them roam free. Or decide that neutering them is immoral or too expensive or something...
If you wanna see some more of Instanbul's cats -> https://www.youtube.com/watch?v=y0dJ4zrTopU (no affiliation)