because it was never vital to survival to have fewer bones. evolution is a term that does a ton of conceptual heavy lifting but has sadly been warped by ignorance and a lack of understanding of the nuance of what is really happening.

somehow, life existed. we dont really know how it got started but it did. due to the laws of physics and thermodynamics, some life died and some life lived. that went on for a long ass time until today we see the stuff that has survived to now. evolution just says life changes over time because some life dies without reproducing and some life reproduces before dieing. thats it. its a statement about the effect of survivorship bias over the long term existence of life.

so why didnt animals evolve to have fewer bones? because it never mattered enough to happen. things kept on surviving with the number of bones they have. we can look at them and say their body would function better with more or fewer bones, or different chemistry, or different soft tissue, or whatever you like. but none of those things mattered enough to happen, or if they did happen they werent better enough to change anything at the time. but also animals do have different numbers of bones, just not so different.

i wish i could describe this better, but evolution isnt an active process. if you have to think of it as a thing, keep in mind that it produces only what barely works. every adaptation we have was developed by the deaths of countless individuals, so we only have it because at some point it was necessary, or so benefitial it couldn’t help but propagate. having way less bones was never either of those things.