Someone once explained it to me like this:
Let's say 1% of the population commits crimes.
Let's also, for the sake of argument, say all crimes are committed by black people.
Of the general population, 10% is black.
If you profile all black people, you will indeed profile the 1% of the population that commits crimes, but you will also profile 9% of the population that does not commit crimes.
And before you say "not the entirety of the 10% black population is profiled": maybe ask some black people about this. It's not 100% of the black population that gets profiled but certainly a very significant percentage.
And even if it's just half or even a quarter of the black population that you're profiling, you're still profiling a lot of innocent people.
This is the reason why profiling is bad.
(and note that in reality the figures are even worse).