Probably cos some people are just dumbass idiots who've never been taught, or learnt properly on how to do it.
And people who don't know that particular stretch or area of road, and as such can't even begin to guess or estimate how deep it's going to be when they get into it.

As for people just driving too fast in heavy rain, standing water etc, they're just f'king idiots, making it a more hazardous journey for the safer more knowledgable drivers amongst us