Except that it involves floating the required mass, spending time moving engies, and finding the time to go back to mexes every 30-50 seconds. Being mad that proper planning gets you an eco advantage might as well as mean removing tree groups so people properly planning around them don't get rewarded for their "apm" as well.
There are times where it's worth ctrl+k'ing mex, and there are times where it is not worth the additional mental overhead. If it is always worth it in your games, you're playing big teamgames where aggression is not a concern and you got like 10 mex to babysit.