There's been much virtual ink and video play time discussing the phenomenon. Just do a search "why are designs so boring?"
My personal slant pins it on the cult of minimalism. I realized we were effed when Lufthansa went with their incredibly dull and depressing livery. Most people in the planespotting world disliked it (1), while designers were falling over themselves gushing about the elegance, clarity and simplicity of the brand (2). My UX designer even used them as an example to emulate (we disagreed on many things).
Also, designers, like most people, are inherently uncreative. A new trend will start, and people will follow. Just look at how every AI project has been trying to shoehorn "Q" into their names these past couple of weeks. Or how everything "smart" had to have a lower-case "i" in front of the brand for a long while. I'm starting to see the backlash against minimalism more frequently, hopefully it'll hit design schools soon and the next home run brand will move away from extremist minimalism
(1) https://thepointsguy.com/2018/02/lufthansa-new-livery-boring...
(2) https://www.adelahaye.com/blog/2020/2/11/feeling-blue-luftha...