Why can't the levator muscle be strengthened with exercise like any other muscle?

In most cases of eyelid ptosis, levator muscle weakness is not the problem. The problem is the fact that the muscle attachment to the eyelid has slipped and that no matter how strongly it contracts, it will not fully lift the eyelid.

I can make this analogy:  lets say you have a hugely powerful bicep muscle. but if your hand isn't holding onto the weight, no matter how much the muscle contracts, the weight isn't going to be lifted off the floor.  Same thing with levator muscle and the eyelid.

Hope that helps

Blepharoptosis, or eyelid sagging has many causes. If the problem were as simple as the muscle being weak, then exercise would help. Problems with the nerve, the muscle, the muscles attachment and restriction by surrounding tissues can all play a part. A proper diagnosis is key to the proper treatment. Seek a qualified surgeon with experience for an evaluation.