First I think it's important to differentiate that MMA & kickboxing/MT are different sports.
There are many more avenues in MMA to deal with leg kicks than in MT/kickboxing - because there are more ways to deal with leg kicks you aren't going to see checks as frequently as you do in kickboxing/MT. This is one reason you don't see checks as frequently.
You also have to weigh up the fact that everything is determined by risk/reward - because MMA & MT/Kickboxing are different sports the risk/reward for each technique will differ. Anyone smart will go for techniques/applications that offer less risk for more reward. That is another reason you also don't see checks as frequently.
Also I'll add checks aren't the only way to deal with leg kicks.
Off the top of my head in MMA - you can pretty much deal with leg kicks by:
a) Moving out of the way.
b) Eating the leg kick to deliver a punch right down the pipe.
c) Grabbing the leg kick and initiating a takedown
d) Check
e) Grabbing the leg kick, lifting up the leg (putting off balance) & throwing a punch.
f) By having superior boxing (pre-emptive counter to leg kicks)
If I was to rank the list above for the most effective way to deal with leg kicks in MMA - I'd probably put checks towards the bottom. Why? Because I'm ranking it based of risk/reward for MMA.
Eating the leg kick and landing a strong punch in a 4oz glove, grabbing the leg kick/initiating a takedown & gaining top control, grabbing the leg/lifting the leg and throwing a strong punch again in a 4oz glove, moving out of the way and having superior boxing - provide a much better risk/reward ratio.
It's not that checks aren't effective - they are. It's that there are more ways to deal with leg kicks and checks are much more situational technique in MMA than in MT/Kickboxing.
For example if you are a superior grappler - would it make sense to check a leg kick or grab a leg kick and initiate a takedown & go to an area of strength for yourself?
Or if the guy/girl you're fighting is a better kicker - would it make more sense to try your best to check the leg kick or try to grab the kick, close distance and enter punching range by throwing a shot down the pipe?
Then you have to remember how often MMA fighters going to throw leg kicks compared to MT/Kickboxers. That's also why you don't see as many checks.
Checking is one of many solutions in MMA. MMA is also much more situational than MT/Kickboxing precisely because the ruleset allows for more & because their are so many backgrounds/styles in MMA. This is why MMA fighters don't check as often.