Some versions of polymorph do include healing. Polymorph Self in AD&D, for example, heals the caster 1d12 points when they return to their own form.
I suppose it comes down to a few factors, primarily:
1. Game balance. Is it good for the game/does it support good play if this spell (which is already often very powerful/flexible) ALSO gives healing into the bargain?
2. How do you conceptualize magic in your game? Is this within the framework? Do wizards REALLY have an advanced knowledge of biology and anatomy with that fine-grained control, or perhaps are they selecting a general shape, and something inherent to their body, soul, spirit, or the magic naturally preserves their general state of health from one shape to another, just as it preserves their memories and mood?
3. How do you conceptualize injury? Is it purely physical? In D&D, especially after level 1, Hit Points are normally only fractionally "meat points" indicating physical injury. They're mostly "don't get hit" points, indicating stamina, skill, luck, possibly divine protection, and the general ability to avoid a telling wound.