Many make that decision foolishly but some do so wisely. There are too many PC manufacturers, it is hard to know which can be trusted. Not many people is in a position to know what is the difference. Since it's always possible for someone to Not everyone can do that.
Nobody can device a 'absolutely secure' system.
However with so many dedicated crytoanalysts and cyberpunks in the world any weaknesses will be discovered sooner or later. The experience with certain browser and many OS's just prove that. So the question is whether that is an 'acceptable risk'. Well, personally I would not use a mission critical system that I have assembled and programmed over the week-end.