Um... because their lines and story arcs are mostly written and directed by white ppl
Remember when Falcon said "I'm just like Cap but slower and weaker!" with such pride and conviction to everyone?
Or the whole " on your left" which was just meant to make Cap look cool at Falcon's expense.. letting audiences know what the deal was from Sam's introduction... which is
not how you sell a new (black) character as cool as far as first impressions...
Or the 3rd acts in the last two ironman movies both having moments where Tony reveals he either could've beat all the bad guys
without Rhodey (the dual red lasers in IM2) or that he
will beat them without Rhodey (all the extra suits at his command in IM 3). So... basically Rhodey is useless.
Idris's whole role was just to be... noble.. and the doorman. Cyborg's problems are more deep rooted than movie writers and directors, but essentially the same swagless shyt. Mr Terrific actually has swag... but they apparently made up a gay limp wrist brother and gave
him the mantle in the CW show...
This is partly why "diversity" (or just more black people) behind the camera is just as important as having black people in front of the camera.