They’re satisfied with the picture of the world in their heads, and looking into things might prove it wrong and then they’d need to go through the effort of changing it, and that’s a hassle, so they don’t. And then you do, and make them do work, which is annoying.
It’s why “don’t shoot the messenger” needs to be a proverb instead of just common sense.
All software gets tested. Some is just tested before release.