Because people are habitual, and mental load increases when you have to learn the UI again every update. Like if someone decided to change all your pots and pans every few months, it's harzadous for cooking.

For the same reason you can keep the interior design of your house the same for decades. Also, why not? It should just be a UI theme, decoupled from actual functionality.

So you don't have to put up with constantly changing UIs that don't improve anything, but require effort to relearn?