It is not something we figured out is ti something we decided.

We needed to find a way to write down something like first add two and three and then multiply that result by five. and we decided that (2 + 3) x 5 would be the best way to write that down.

We could have decided on another system but that one is useful in many ways with how numbers work.

The important part is that we all agree that what we write means the same thing.

The details are far less important than that we are all on the same page.