This is deeply rooted in the human psyche because it has been a premier pest throughout human history. In many warmer climates still today, it is a constant companion and, in many areas of the world, no attempts are made to eradicate it because this is difficult to do without expensive chemical applications that may not be feasible on a wide scale in poverty-stricken areas in particular. So all human beings have had lifetimes living with cockroaches as an ever-present feature that becomes, after a time, emblematic of their general helplessness in the face of many such scourges and the reality they have limited control over their own environment and their enjoyment of life because inevitably there will be intrusions that are disruptive.
The presence of a cockroach has come to represent poverty, encroachment, and deep stirrings from times of great fear as children waking up at night and having cockroaches on their person. It is most unpleasant to have large numbers of insects like this ever-present in a dwelling. The personal karma carries many such traumatic memories of encounters with predators and scourges, both. Seeing these creatures stirs those memories within the deep subconscious and adds significant stress as a consequence that will be poorly understood consciously but is very, very real in the experiencing of it.