How did things get so out of control without the residents realising the magnitude of the problem?
It could be because German cockroaches are nocturnal, preferring to feed and breed when you’re fast asleep. And, they are super-fast and tiny, so can quickly scuttle for cover if you have a chance encounter during the night.
Added to this is the fact that German cockroaches reproduce faster than any other residential cockroach. Female German cockroaches can reach sexual maturity at about six weeks, producing a case of up to 50 eggs every 3 to 4 weeks, allowing the population to multiply rapidly.