This isn't true, US studios see themselves as saving narratives from books in a way their Japanese counterparts do not. If the inconsistent continuity of comics were the cause we wouldn't see similar issues with novel series, plays, etc. The money and ego in US cause them to…
Adapting a Manga is far more straightforward than adapting a comic because Mangas don't relaunch or switch teams you don't have to choose a specific run of the character to adapt because 99% of the time the guy who starts is the guy who ends it