Why does: 5x+3x+2x = 10x and not 10xxx?

Because you are not adding the x's .When you add the coefficients, the variable comes along with it.

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