It's true that Hollywood can't find and rely on star power to drive the box office anymore. Tom Cruise is the closest but he's no guarantee; remember "The Mummy". Similarly RDJ has had his fails; Dolittle.
There's probably a few reasons.
Some we get to see as charisma free thoroughly unpleasant arseholes such as Brie Larson or Jennifer Lawrence. In previous generations we didn't see as much of them and what we saw was more controlled. Related to that a disturbing proportion of these people and those behind them are deliberately antagonistic and offensive to large parts of the potential audience. A number of the MCU cast are in this category including Larson (again), Ruffalo and Evans. That's not going to build up a wide audience with affection to the actor/actress as opposed to the role being played.
Then there's the issue that whilst the studios want marketable star power in the context of the current movie being released they don't want it more generally. They want people attached to the IP/character with the ability to swap new cheaper (more diverse) actors into the existing character/IP without adversely affecting audiences. The studios want the power over the actor, not the other way round.
There's another studio driven issue which is "current writing". The MCU is a great example of this. Take Elizabeth Olson (if only!). As far as I'm aware she's never gone out to offend audiences or said or done something terminally dumb. There's a series that Wired does with celebs reading internet questions from cards and answering. It's the one that gave the world Brie Larson's "Is that a personal attack?" clip. Compare and contrast Larson's with Olson's. Olson comes across as delightful. Can't say the same for Larson. Olson is a perfectly good actress and has that old school charm and charisma in spades - look at the early episodes of WandaVision. However..... her character is not written as a human. In her early MCU material we had a coherent backstory and redemption arc. By WandaVision we had an all powerful psychopath kidnapping and torturing a town and the writer tries to tell the audience that this is all OK because she's grieving. By Dr Strange 2 she's a malevolent irrational inconsistent psycho willing to destroy universes in her quest for her fantasy family that never actually existed. "Current day" writers either can't or won't write characters consistently with basic fundamental human behaviour and that harms an audience's ability to empathise with the character and, by association, the actress.
And so to Jonathon Majors as Kang the Conqueror; the character that has to be the big bad to eclipse Thanos for the next ten(?) years. He's not so much Kang the Conqueror as Kang the Constant Failure. He's the Wile E Coyote of the MCU but without Wile E's charm, charisma or character development. Who empathises with this incoherent bozo? And he's already following the Simu Lui example of behaving like an arsehole playing the big shot who's always right and everyone else is wrong as his film (supposedly crucial and pivotal to the future of the MCU) crashes and burns. Yup; this'll end well!
