There's also a survivorship bias as illustrated by something like Batman v. Superman (sorry for infinite Synder content). Snyder clearly planned for a ~180M cut of the film but the studio's lack of faith in it caused it to be cut down to ~150M which creates some obvious plot holes (Lex Luthor's plan is to shoot people with bullets and blame it on Superman?). Morbius was never a "long" movie but it also clearly got cut down due to poor test screenings.

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How about this mental model: superior quality in blockbusters probably inherently tends towards longer movies because directors will always want more of their stuff in the movie and the film needs to be good to give them leverage to include it.