Because just because someone can or could do some physical skill in the past, it doesn't mean they are good at teaching them. Also two other features, McHale is old and a lot of players don't even know the basics about how to be effective post players. Outside of Scola, the best post player the Rockets have had since Yao retired has been Motiejunas which tells a lot about the big man talent coming out of college.
It's easy for a guard to learn post moves, because you're acustomed to creating space before shooting. Big guys don't have the ability to do that, because we're slower and the defenders have much bigger bodies to get away from. My problem with learning post moves is it requires more finess than a jump shot. Combine this with big centers and forwards who tend to have the least dexterity on a team and you can see the problem. You can't really practice very well with a trainer holding pool noodles, because half of the battle is getting into position for a post move. You need real defenders and even then not all centers/forwards defend the same. Some are very physical and you have to fight for every inch of ground while others play a little off of you and try to block. Even when you go up for your floater/hook/layup, there's a varying amount of contact that you have to adjust to. That's why you see big guys miss simple layups and short floaters, because sometimes we expect contact, but don't get it so the shot is too hard. Othertimes, we don't expect contact and get it, which makes the shot short.
