Because a bunch of mathematicians got together and came up with a order of operations so that there would be no confusion when looking at a math equation.

It is this way is because all those math nerds said it should be that way. That's it. Left side of the number line is negative because the math nerds got together and said that is how it should be. Much of math and science has shit like this where 'by convention' i.e. nerds got together and made a decision, something is done a certain way.