I suppose it's because they live in an environment where there's a constant vulnerability to predators and they need to respond quickly. But it's not really the case for many invertebrates and besides many vertebrates too live in such environment.
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I think this falls under this Q&A: https://biology.stackexchange.com/questions/35532/why-do-some-bad-traits-evolve-and-good-ones-dont There's not really going to be an answer besides "that's how they evolved". – Bryan Krause Feb 02 '24 at 20:50