"Because that's the rule" is only half the point. It's the rule for a reason.

Multiplication is repeated addition. It can be unpacked into addition: 5 x 3 means 5 + 5 + 5 2 + 5 x 3 means 2 + 5 + 5 + 5

The same relationship is true for exponents and multiplication - exponents are repeated multiplication: 4^3 means 4 x 4 x 4 4^3 x 7 means 4 x 4 x 4 x 7

Subtraction is reversed addition. Adding and subtracting are done in the same step - last.

Division is reversed multiplication. Multiplying and dividing are done in the same step, just before addition/subtraction.

Exponents (and logarithms, where applicable) are done in the same step, just before multiplication/division.

The only thing that's really convention for the sake of itself is parentheses, because we needed a way to demonstrate that certain parts of an equation had to be solved out of order. So anything in parentheses is done first, because that's what they are, by definition.

There are some really interesting low level algorithms that take place in computers and calculators, to turn equations into long strings of logic gates, in a similar sort of method. Pretty much anything that can be computed can be unpacked like this, further than most of us would ever notice.