Because insect are much smaller. They need to see better because they have predators they are much bigger and will try to eat them. So they need to see movement better than seeing details so they know when an predator will pounce. So that way they can escape. Insects will also need to see movement better so they can find their prey too.
they have multiple little eyes but even so, each one still dosent see very well, which makes sensing movements much easier
I dont know if this is what you were looking for but i hope i helped in some way :)
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