half of it is mindless "I get stronger when I yell", "oh no! now there's an even MORE powerful bad guy! What will we do?!", or "Kyaa, senpai!"

You say this like its a bad thing. I dont always want some epic on the human condition. Sometimes I want to just turn my brain off and watch two dudes punch each other at light speed so hard that one of them is sent flying to the literal moon (or did you think it a coincidence that One Punch Man ended up being extremely popular?).

And its not like anime is devoid of shows with serious topics. Fullmetal Alchemist deals with war crimes, government conspiracies, death cults, and an evil lead by an eldritch abomination. Demon Slayer is about an orphaned child going through hell and high water to keep his only remaining family alive, even though she is technically now a demon. Hell, even the ones that seem like jokes on their face can be serious. Fire Force has significant amounts of fanservice and joking downtime, but the main plot revolves around (spoilers) >! an extremist religious cult mass murdering people to perpetuate their own religion. !<

Which is the main reason Anime is so popular, IMO. It can be serious, or it can be fun, or if they are skilled they can walk the line between both. The amount of American media that can do that in this day and age can be counted on your fingers.