Cockroaches are just plain nasty.
Hiding away during the day in dark crevices they emerge to feast on food left by humans - even the tiniest of crumbs.
The very young cockroaches are coprophagic which means they eat the droppings of their older relatives as they hide during the day. This and other survival instincts make them very hard to eliminate.
The most common form of cockroach are German and Oriental Cockroaches in the UK.
The most effective treatment is by way of gel baits rather than residual insecticide. Residual insecticides don't kill the very young nymphs as these are tucked away usually out of the range of a physical spray due to their eating habits. If residual insecticide only is used for treatment the treatment will generally fail.
The more sophisticated professional use gel baits are readily eaten by cockroaches who return to their hiding place. Sadly (for them) they die but not without leaving droppings which are eaten by the next generation. They die too along with the cockroaches 'friends' who feast on their carcases. Usually floors are littered with dead cockroaches within a few days.
In severe infestations a combination of gel baits and ULV (ultra low volume) treatment may be needed. This is where chemical is mixed with a very fine mist, far finer than that of a conventional pressure sprayer, which drifts into the cracks and crevices where the cockroaches are hiding.
It is important the ULV treatment is left for at least an hour or more as cockroaches can hold their 'breath' for up to 30 minutes.