In my case, the roaches were coming up from the sewer pipes up through the sink because the pop up sink drain stopper was damaged and could not close and the outlet pipe doesn't have a water trap. After I replaced the sink drain with a normal strainer and plugged the overflow hole there were no roaches any more.
Another case was the kitchen drain grate cover which was pushed up because accumulated cooking fat and other stuff was blocking the pipe. After unclogging the pipe, roaches couldn't climb up anymore because the normal water level and the grate cover will stop them.

I've seen a roach coming out of the shower grate which could not properly close in a friend's house, so thats one way roaches can get inside, that had to be fixed by changing the shower drain to an insect proof one. Condos also have to worry about air vents and improperly sealed piping.

I don't believe most houses should have roaches if they are kept clean and well maintained.