It is about magazine cover. And the more I think about the more it makes sense that it is decided by editorial. Magazine is their territory after all. It also explains why they are so generic. But volume covers might be very different even in one series. Beside you dont need to know much about manga to make design for mag cover. But you need to know a lot to make particular volume cover. Does that means designers have to read manga ongoing? Maybe young authors do have to consulate with designers but I cant imagine someone beside Oda came up with 92 volume cover.
Its common for the publishing industry as a whole makes complete sense.
The cover not only has to well represent the content of the book, but also needs to be something interesting enough to pull in sales.
Sometimes the mangaka themselves won’t have the best idea for those sort of thing. It’s likely that a team would say something like “Hey I know you want to put Mirko on the cover, but the highlight of this volume is the Twice and Hawks fight. So you might want to make the cover something illustrating that”.
And also, illustrators and graphic designers aren’t jobs that always go hand and hand. There are some illustrators that can make a great design layout. But a lot of illustrators don’t have a good concept of logo making, and creating the overall layout of a book. Which is where a designer comes in.