Waste/garbage created by humans is probably an easy source of food for them.

Plus rodents survive in hiding. If they come back up to the surface to seek food, then they will become targets of owls, foxes, etc. But Thule they live in the sewers, that gives them protection.

They’re hardy scavengers that benefit from human settlement and food waste.

Benefit in the most horrific of ways. I once watched a cockroach eat a dead cockroach. It was very... enlightening.

Welcome to nature. We've civilized misery, but it's misery nonetheless.

Cockroaches eat almost anything, like rotting wood, glue, toothpaste, dead things, food waste, etc.

And they can also go about a month without eating anything.