It's just a convention, you could instead write out each operation explicitly with brackets, this convention allows you to not need the brackets. The convention is far from perfect, leading to viral math problems of type 6 / 2(2 + 2) = ? Does that translate into this:

   6
--------
2(2 + 2)

or does that translate into that?

6
- (2 + 2)
2

You can argue about the right answer until the end of the universe, but the fundamental failure is that there is confusion to begin with, meaning that PEMDAS is not clear and unambiguous enough. In notation with the horizontal bar you notice there is no confusion, none of them are ever going to become viral math problems people fail to answer right.