It was much harder to spray apartment complexes with hundreds of units and treating for roaches in every single unit was prohibitively expensive (or my boss was a cheapskate, I don’t know), so we’d only do targeted treatment where we had seen roaches and surrounding units.
Single family homes were much easier to treat. I strongly suspect we got roaches in our single family house when a home care nurse brought them with her, along with a bed bug! I almost died when I saw that monstrosity, luckily we avoided an infestation, those are 10x worse than any cockroach. You do not want bed bugs.