system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there no large animal with more than 2 eyes but spiders have a lot of eyes?
gpt
The last common ancestor of all vertebrates had two eyes and that's worked well enough. No mutation that gives more than two eyes has given a significant survival or reproductive advantage, so we've stuck with what works.

The arthropod body plan is a bit more flexible when it comes to numbers of things. You can have anywhere from 6 to 100 legs, plus 0 to 4 wings, 2 simple eyes, compound eyes, or 8 or whatever.