The problem with it is that people do not usually check if its actually provably fair or not because it can be quite complicated, various casinos were found to be cheating even though they were using provably fair systems, in theory.
I have to agree that not all gamblers use the system to check their bets. I'm not quite sure why but it seems something related to "trust", once gamblers trust the site then they wont really care about their bets no matter win or lose. The other reason could be about knowledge, maybe some gamblers does not even aware how provably fair system works so they do not care about it so their main idea is to gamble only without worrying about being cheated if they play on non provably fair sites.
I agree, even I sometimes do not verify my bet when gambling

Not because I don't want to, maybe because of trust and sometimes it's tiring to check your bets from time to time. When I was new in gambling I don't really care about this as long as I am playing.

Of course we don't check every time whether the outcome was fair or not. What is important is that we
can do that any time we want. I just did it for my few last bets on PrimeDice, and below are the results:

^^These were my bets

^^ And these are the results of verification from the site dicesites.com/.
It appears that I made 78,868 bets with the previous pair. That's how much I trust PD.