Most people don't know how to do it correctly. Even before I outlined what he can do, people have won numerous sets through planking or Dair camping to planking to run out the time. I also explained why it doesn't win tournaments awhile back (most people associate winning a tournament with a strategy doing it in Grand Finals even if they have never done it in their life lol, but exempt scenarios where the strategy is used in most sets barring grand finals.)
The first problem is that most people who play MK will "reserve" playing gay unless absolutely necessary. Most MK's, from M2K to Lee Martin to even Dojo, fit that bill. M2K can beat what, 95% of the Brawl community without playing gay, and the other 5% when he decides to. You have the general lack of gayness until things get "close" in Brawl; how many people do you know of that will actively try to time someone out at or before the 6 minute mark? No one has that fervor for playing gay, shamefully not even I possess that kind of zeal and vigor. Once things are close, and the gay is turned up, people say "Oh well he just won cause he's better". I rarely hear someone say "He won because he planked or ran away", and hear "He's just better" despite the tactic clearly shaping the game significantly. If people associate winning with always being better, no tactics coming into play, then of course most people would fail to say "Hey wait a minute, that's a bit ********".
The reason you don't see MK in Grand Finals planking? Because it tends to be M2K vs whoever, and even when it gets Nasty M2K tries not to whip out any time wasting shenanigans unless it's crucial that he does. Even then, I've seen him stone faced approaching going against his urges to plank and fight people straight up. The other few times, when it's MK vs MK, it just be a battle of who's better at planking/stopping it. Most people don't want to press their luck, and don't try it. Despite doing it in the previous tournament sets against non MK players, you might rarely see it in Grand Finals unless people "want" to do it.
If people actively, ACTIVELY tried to play gay with MK, and then people were doing well against it, you could go "Oh ok, no problem yet". But we're barely scratching the surface of gayness at this stage IMO. There are so many timeouts, so many plankings, so little time...