It is a little more complicated than that. First off there is the long history of white people playing POCs for the sake of mockery. That must be taken into account when you think about why white people playing POC. Second, there is a need for representation by POC's. Representation is very important, and POCs are still very under represented in media, with POCs still showing up more often as a villain than a hero, or as a token character with no real arc or development.
Everything being equal what you said would be right, but things are not equal. POC's still lack representation in media. There are very few POC writers to write POCs in authentic ways, so most of the time when you see a POC on screen they are still saying the words and thoughts of a white man.
We are still working on fixing the mistakes of our past. It is going to take another generation or two to really get to a point where it no longer matters.