Change is done this way for two reasons.
- Suppliers require commitments. And a manufacturer requires commitments from suppliers. This helps keep per-unit costs down, but it constrains your ability to make running changes. Tesla has fewer issues here than most due to their vertical integration - they are less dependent on suppliers.
- Cars are (expensive) fashion accessories. And fashions eventually go out of style. Tesla has had a remarkably long run with the Model S. The facelift helps a lot. But eventually, sales will slow and it will no longer make sense to keep building the Model S. Tesla will need to design a different vehicle to drive demand.
But I agree - I love that Tesla can and does make changes as soon as they are able, without waiting for "the next model year" to introduce them.