Recency bias. Fallout is great but it's also a dying breed of movie with no competition. Like fellow action star Keanu Cruise is one of the last action stars from the tail end of an era where action stars alone could carry a movie. He's one of the last we got still active, so that's why he's always in the conversation.
He's as daring as they come and he might be a better actor than any of just about anyone else pigeonholed into "action star", but I think the latter actually works against him. To be a truly great action star in my mind you have to be intense during the action, but (for a lack of a better word) dull and sincere outside of it. Just look at Keanu. He's not known for doing insane stunts, is a really limited actor, but people love him. Cruise is not blue collar enough feeling, he's too sharp, too handsome, too good of an actor. In terms of accomplishments, as others have said, has he really beaten Arnold in physical presence and Chan in terms of stunts, and the movies that made them? Their movies that defined the genre forever? That's real hard to argue for.
Maybe you can make a better argument for him being better than Stallone and Willis, who have spotty records outside of their key franchises, but even then those guys get by on the same x factor, their perceived presence as these working class blokes/down on their luck dudes in shit situations you can really root for. That everyman relatability (even if in fact, Stallone and Willis are known to be real life assholes). Cruise is just missing something crucial in that department. He's too intense, all the time, like there's no off switch, and at least I've never felt like I can connect with him as a person or a character.
EDIT: The Rock has this vibe I'm talking about, but his filmography is pretty shitty overall so that keeps him from greatness.
Arnold is the de facto "80's action star", but man Tom Cruise's output over the last 30 years has been insane.
He has so many great action movies. All the Mission Impossible movies, Edge of Tomorrow, Oblivion, Jack Reacher, War of the Worlds, Collateral, Last Samurai, Minority Report, Top Gun. The list of quality movies is unbelievably long.
Of those movies only Mission Impossible series, Collateral, and Top Gun would I consider among "great all time action movies". The others are decent or even great films but not really that amazing as action films go. Certainly not in the breath as the classics of the 80s and 90s.