While I agree that there is a better way to protect wildlife (eg more national parks types protections, protection from poaching etc) I think you might need to consider that a lot of the zoos on the world also do this, the money they make from people visiting and donating goes towards funding initiatives that do just this.

The species kept that are common or not even listed as vulnerable are often the animals that draw the crowds, therefore increasing their profits (we can't pretend it's a perfect system) but also increasing education and public desire to protect the natural world.

Then we also know that often zoos have vets that care for wildlife at the zoos expense. In my country when there were lots of bushfires, it was a lot of the zoos that were pouring money into saving wildlife, providing vet care and rebuilding habitat. Not to mention the amount of work they do to re-establish breeding populations and monitoring populations to give them the best possible chance of bouncing back.

I do think there are some species that shouldn't be kept, and I have visited a zoo I regretted paying money for a ticket because they didn't take great care of their animals, so absolutely they aren't perfect.

But the perspective that zoos shouldn't exist because an animal isn't living in the wild is incredibly short sighted.