A driver will only occasionally turn their head to watch blind spots. They might forget.
That would be extremely bad driving habits. Keep in mind that in most places if you don't check your blindspots when changing lanes during the drivers test it is at least a significant point loss and likely an automatic fail.
I took a refresher-course after moving to Belgium, since I hadn't actually been driving for some years, and I wanted to also learn about differences in laws - the instructors basically told me that they never bother telling students to check their blind spots, because "why bother"....
Since moving to Belgium, I've largely stopped riding my bike. Experiences like the one above, and people's driving-habits in general here, scare the shizzle out of me.