meaning - As good as it gets- grammar

I do know what "as good as it gets" means (in my language, we say "it will not get any better").

However, I do not understand the grammar here:
Firstly, does the "get" mean a change of state here?
Secondly, why the present tense? It can get better in the future, can't it?

Could I say "as stupid as he gets?" What would that mean?

EDIT: I have found this one: I was expecting there to be more people at this dance party, and I was hoping there would be more nice ladies/gentlemen for me to meet. Is this as good as it WILL get?
If this is a permanent concept, why that person use a future tense?

Answer

When the context is absolute ("as good as," meaning "the best"), then the present tense is used to indicate permanence. "The best" is permanent because it is never better or worse; it always is what it is. There is no future or past tense for an absolute condition, because an absolute condition is considered not to change.

So your confusion comes from this: You are thinking of some thing or some object as "becoming" something, but the structure of the phrase does not refer to any particular thing: it refers to the absolute state. The absolute state of "bestness" is a concept that never changes, so it is referred to in the present tense.

Here are two more ways of looking at it: 1. You were trying to correlate "get" with "become," and that has some validity, but it's hard to see the correlation when you just correlate the present tenses. Try taking your translation ("it will not get any better") and put "become" in there, and that should make sense to you ("it will not become any better"). 2. Consider that there could be another word in the phrase "as good as it gets". It could say "as good as it ever gets." Ever is implied; we don't need it, but if you put it back in, doesn't that make more sense to you?