That reason is people. Journals don’t just create themselves. Lots more happens to make it so than just having an email inbox to accept submissions.
You need to be able to fund the enterprise, and often you want bigger/popular journals to subsidize the really esoteric stuff that’s important intellectually to humanity but still requires a base number of people to run it independent of the sales. It would also be nice for these people to make real money for their time so people who are good at it could make a career of it.
Elsiver etc are a different beast as they’re companies disconnected from a larger university. So their self-preservation goals are quite different, and accordingly, their appetite for organizational profit versus covering expenses.