It's not really just a MCU thing though, both DCEU and the X-Men at Fox are pulling equally big names on their rosters. So did The Dark Knight Trilogy before that, so did both previous Spider-Man incarnations.
Even farther back, the original Batman movies 25 years ago had Jack Nicholson, Kim Basinger, Jack Palance, Michelle Pfeiffer, Christopher Walken, Danny DeVito, Jim Carrey, Tommy Lee Jones, Nicole Kidman, Uma Thurman and Arnold Schwarzenegger, and that's not even counting cameos like Billy Dee Williams, Jerry Hall, Drew Barrymore, Debi Mazar, Ed Begley Jr, Paul Reubens, Vivica Fox, Elle McPhearson, John Glover, Rene Auberjonois... if we're talking relative star power those are still unmatched.
Heck, even the 1978 Superman had Marlon Brando and Gene Hackman.
So I really don't get why you're all singling out MCU as some never before seen draw for big names...