Why does Hollywood make mixed actors play as black characters?

At this point in the US most African Americans have at least some white ancestry in their families.  If all the DNA testing that has become common is credible most people in the world have very diverse ethnic origins. This is not just a US phenomenon it is world wide.

Hollywood isn't about reality it is about selling fantasy.  Thus they used mixed actors because the lighter skinned people [mixed] are "considered more "attractive" than darker skinned people. 

They don't 'make' them do anything.
If the best actor to audition happens to be mixed race rather than fully black, that's who will win the role.

hey! were they mixed with Asians?

They are black. You sound British. Like Satan, come to steal kill and destroy. Divide and rule is try to destroy.

They hate you.
That is the only reason.
I agree with you that no "mixed" actor should ever be given a role in Hollywood. I believe you are right when you demand that the races be kept pure.

To balance that fully Asian actors play mixed characters 

because they ran out of black face paint