Maybe the kill rate is so high it doesn't make sense to do the surgery on an animal that may be put down in a couple weeks.
That was the case at the shelter where I adopted Zephyr. (If you substitute "days" for "weeks.") They didn't feel able to put the money into individual pets when the majority of them were having to be put down in less than a week. Not saying I agree with this, just stating facts, but they didn't do anything medically for the cats they adopted out besides provide a voucher for the first vet checkup. I paid for everything else and arranged all his appointments, including his neuter.
I didn't mind paying - my cat, my responsibility - but it did
make me worry that other people are probably sometimes adopting cats and never doing anything for them, including spay/neuter. I also worried that some of the cats might be carrying contagious diseases out of the shelter with them, or going to homes that will dump them if they turn out not to be healthy.
Honestly, though, I'd be hard pressed to come up with an easy solution for that shelter, so I can't fault them too much.
As for vets... like people in any profession, they're all different. Some are very profit-motivated and would never do anything they weren't exorbitantly compensated for; I've known some like that. On the other hand, I've known others who already provide their services at the lowest possible cost, who would bankrupt themselves and be able to help no one if they tried to do more free work on top of it. And there's everything in between. It just depends on the vet.
Some shelters keep cats in cages to reduce the spread of disease; a large number of animals in a small space can be terrible for contagion, and minimizing their physical contact with each other can keep them healthier. Some cats feel safer in a smaller space when they're in a strange place, and it prevents them from fighting or attacking each other when they can't be supervised. Again, I'm not advocating it as a great solution - just saying this is another area where there isn't always a simple fix for a crowded, overworked and understaffed animal shelter.
I think it's great that you're volunteering at a shelter! Thanks for helping the homeless pets in your community!