Well, I asked the same thing Lord, and this is what i got from the answer:
It's because Skeletor is a insanely powerful wizard...which He-Man has problems against really powerful magic.
He-Man doesnt need his strength in every fight......sometimes he throws car sized rocks at people, other times he throws planet sized rocks at people.
All depends on who he's fighting.
If he faught Wolverine, he would be just a little stronger than Wolvie.
If he faught Hulk he'd be stronger than the hulk.
But prolly get his ass handed to him by Dr.Strange.