Cockroaches vary dramatically in size and not just by species. Unlike most insects they do not start their lives as larvae they hatch from an egg as a fully formed baby cockroach. And they are tiny. So small they can get in the back of a microwave vent or into cracks under caulking in bathrooms. They are so tiny they can infest inaccessible areas of the house. While this may be a severe problem when trying to accomplish a domestic treatment it is no issue for a commercial-grade treatment with its multi-chemical process.
Getting into strange and hard-to-reach places is very much a cockroach talent. They live in sewers and can come up through drains and into toilets. They are capable of flattening out their entire bodies to be paper thin like a playing card and hiding under rugs. Trying to stop them with a shoe is difficult and may not even kill them. One of the most problematic places they can enter and it’s problematic for a very specific reason are ventilation shafts. While these places are usually hard to enter they can crawl through vents and use them to get around the house.
The reason this can be a serious issue is that cockroaches go through a long moulting process as they age and grow and it generally does not stop until they die. This moult they shed off is coated in dangerous bacteria from sewers and other dirty places cockroaches go. The moult will come off them in tiny flakes that can float in the air and be directed through the ventilation and pushed into the central areas of your home like a waterfall. This intensifies the effects and can result in dangerous respiratory infections and worsening symptoms in already existing respiratory illnesses.
You can try treating them with domestic methods but it often does not kill the full population. Insecticide dust is effective against adults but babies tend to hide in places like microwave vents. You would have to treat the whole house from top to bottom to get rid of them and with domestic tools that may not be possible. You can spray permethrin all over the place on walls and in cabinets and place gel bait all over the house but you will never be able to treat it completely and they will always find somewhere to hide.
If you call a professional they can do that. They can spray the inside of appliances and motors using commercial-grade pesticides that will kill everything. From babies to adults to eggs. The cost may seem larger than you wanted but trying to treat it yourself will take years and you may never really get rid of them. The health dangers are too severe to risk that possibility so you will have to get professional treatment eventually anyways. Why not get it first and skip the nightmare of domestic treatment?