Cockroaches are pretty impressive and live a long time, at least as far as insects go. Fruit flies measure life in the order of hours, but roaches can live for a year or more. I suspect you need a more complex nervous system to assess and survive in an environment as a prey animal for long periods of time, whereas fruit flies can likely just brute force it.

Fruit flies can live for well over a month. The hours belief is a myth.

This question makes me wonder if larger bodies have more complex physical/mechanical dynamics to solve for. Do physical laws become harder to get along with as your body plan expands? I wonder if that’s a factor.

That’s not how evolution works. A cockroach might have 4x neurons because it just does.

Brain size usually correlates with body size, and cockroaches are much bigger than fruit flies.

I believe it’s something to do with operating the larger body.