Why doesn’t the brain heal, but compensates instead?

Keeping it simple, when skin is damaged, other identical cells grow and replace the damaged or removed cells, creating skin that looks good as new.
The brain is different.
The brain functions based on the interconnection of millions of cells called neurons.
Unlike skin cells, each neuron has an individual function that is learned, based on the circuit of neurons it lives in.
When a neuron is damaged, oftentimes it is not replaced. The damaged neuron does not “heal” or return to 100%.
The ability to regain function, or “compensate,” depends on the health and growth of the undamaged neurons around them.
The functionality can be regained.
However, sometimes an unhealthy brain will compensate poorly. This can look like persistent symptoms.
When therapies are designed to help the brain compensate appropriately, often results are expedited and greater success is achieved.