Convention.

We could fully parenthesize everything, but it gets annoying to write polynomials if you have to put parens around everything, and it turns out that polynomials are super useful in math, so we set up a convention to make them easier to write purely for convenience.

If it happened to be that we most often wanted to do addition first instead of multiplication, then we would have set up the convention so that addition has a higher precedence so we wouldn't have to write a lot of parens for that thing we do all the time.

That's literally it. Mathematical notation simply exists to be a concise way of writing what we do most often to save us writing. It's nothing to do with math.

This is why it's super annoying when people insist that some equations like 6/2(1+2) are ambiguous. Literally the entire reason we set up these rules is to make sure that there are no ambiguities, so unless you're willing to accept that these super smart people that set up these rules just did a crap job of it and you found a flaw, the situation here is simply that you just don't know the rules they set up.