Or maybe some of them just hang around to give the birds and frogs something to eat?"
Of course they did evolve, into well over a million known species today, some of which fly, some don't.
But the basic insect body-plan -- exoskeleton, six legs, small brain, no lungs, heart, veins or arteries, etc. -- can only stretch so far.
Here is one of the largest bugs to ever fly -- two foot wingspan, lived 300 million years ago, when atmosphere's oxygen was higher than today: 