Probably bc they don't care about the franchise lol and just want to use it as a stepping stone for their own agenda. They know it and we know it; we're just playing nice about it because this is a kid friendly message board. What other reason explains why it's a "problem" if T'Challa is "shoehorned" into Avengers End Game but oh by all means, shoehorn in Namor, Storm, Dr. Doom, etc..!
BP 1 got so many open plot lines (intentionally left for the sequels) and so many in-franchise supporting characters and villains that still need to get off the bench, that we at standing room only. 1 of those 3 will get 1 of the 2 credit scenes, i imagine.