Because the hook shot is not an efficient shot, largely.
Hook shots are much more difficult than other "close-in" shots; while they solve the issue of the defender blocking you (somewhat), they are harder to aim.
You say "a player who could reliably make a hook shot could crush an opposing team". Sure! But that player doesn't exist.
Looking at this analysis of hook shots from last year, nobody makes 60% - and only a small number of players even make 50%. Sure, nearly 60% is pretty good (it's better than most shots from anywhere other than just at the rim), but it's not earth-shatteringly good; remember this chart? 1.2 EP is your average three point shot from the side.
So maybe Robin Lopez should take some more hook shots; but Robin was also making 33.3% of his three point shots, so it's not like he was leaving a ton of points on the table. And you can't ignore the difficult of getting the ball in to the player to take the hook shot, nor the likelihood that, were Robin (or Nikola or Brook) to start taking more hook shots, defenders would play them differently and defend the hook better.
I'd also slightly object to your premise; "big men" aren't just rim protectors. Plenty of big men are also shooters from the outside - see the Lopez brothers for example. But that's the thing - the ones who might be able to make a decent living with the hook shot are also the good outside shooters.