Because there’s no profit in it?

It does, in a roundabout way: people that exercise more go to the doctor less and take less medications, so insurance companies could profit a lot by encouraging people to go exercise.

You’re not wrong, but you can’t patent physical activity which would make it the least attractive option in their partners portfolios.

Profit was already mentioned.

Another reason is, medicine gives you (the doctor) something to point to and correct if things don't work. It's "mono-factor". You are lucky if you need more than two variables (dosage and schedule).

Something like exercise, or a surgery, depends on too many variables to make work exactly the way the doctor wants, it's "multi-factor", and if it does not work, gets you plus another doctor or medical expert in another, more applied field, more involved with your time when there are other cases on the pile.