Its what they wanted. Non variation and same tone for all their different characters outside of Cap and Black Panther. Their version of Hollywood is homogenous and family friendly.
Case in point, the Russo Bros who directed the Captain America films, who moved away from the predictable MCU plot, is directing IW (a decade in planning).
Meanwhile those other directors who made those other MCU with cheesy humor ain't doing anything major outside of sequels.
So whoever makes that argument that 'fun' factor in the typical movie should ask, is "why isn't their directors widely known like the Russo Bros or Coogler? And why they ain't doing a massive project like IW if they were so successful?"