because you are adding an amount of the same variable. They are like terms so you would combine them. If it was 10xxx it would be 10+3x. But, 5x+3x+2x is 10 because 3x+2x is 5 and 5x+5x is 10x. You do not add the actual x's together, just the amount that is shown.
Think of it as this way:
5x = x x x x x
3x = x x x
2x = x x
in total, there are 10 x's
