The main reason for keeping AI static is to allow them to be certified or rolled back (and possibly that the companies can make more money selling fine tuning) — it's not an innate truth of the design or the maths.
While those are good reasons to keep the weights static from a business perspective, they are not the only reasons, especially when serving SOTA models at the scale of some of the major shops today.
Continual/online learning is still an area of active research.
