Because they lack even a modicum of creativity and they know that their pastiche self-insert social agenda megaphone characters go over about as well as a turd in the punch bowl at a high school dance (see: the New Warriors reboot that didn't get off the ground). They need a way in to be able to blast their message to as many eyes as they can and they can't wait for their trash to gain popularity, so they co-opt existing characters into their agenda (Iceman, Tim Drake's Robin, Squirrel Girl) or replace them with race-lifted Mary Sue versions that are supposedly better than the originals (Riri as Ironheart, Sam Alexander as Nova, Kamala as Ms. Marvel, Amadeus Cho as Hulk, et cetera).