They can only make one or two movies at a time. To put it another way, using those big names is a way to bank roll the films that introduce and build on those characters and allow them to rise and fall as they may. If Avengers wasn't successful, I doubt Captain Marvel or Panther would even have been considered.
And as much as Hollywood would like to deny it, the US and the world just isn't as liberal and accepting as they are. The country is far more tolerant than it was, but aside from extremists on both sides, most of us just flat out don't care, and be just as happy if people stop trying to shove acceptance down our throats as much as rejection or violent opposition.