Often feral cats can't be rehomed due to behavioral issues like dogs can, they just don't have the mental framework for it, and there's so many kittens available that it's often not worth it to try.
They also pose a pretty gnarly threat to the local ecosystem, particularly birds (and children's sandboxes 💩) because their group behavior means they want to hunt more food than they need. That's why they bring you their kills, be it a cat toy or a real creature, and why they really don't like the food being empty, even if they don't eat it all like a dog would. They want a surplus and to make sure everyone gets fed.
Finally, a lot of no-kill or donation based shelters are extremely loath to cull cat colonies because it can reduce donations that they need to do other work, because no one likes donating money to killing cute kitties, even if it's necessary and that's not what's really happening. (Taxpayer funded animal control doesn't have that issue, but they might be stymied by other factors).
What a lot of places do as a result is to capture the cats, sterilize them, and then return them to hopefully keep the problem from getting worse or maybe even get the size of the population to drop. Others just remove their shelters and disperse the colony, and try to get anyone who's been feeding them to stop.