This is a misunderstanding. For simplicity’s sake, mwi people say that the universe “splits” into one child universe for each possibility- but that’s not what mwi actually says, just the pop science simplified layman version.
In actuality just as the quantum wave function is a continuous, analog wave there is a continuous analog sea of possible universes, and some have greater measure than others. And some are more correlated than others. The discorrelated ones appear to be separate from each other. But they’re all part of the same multiverse.
Think about it this way: we have no trouble conceiving of the universe having multiple possible futures, but only one past. That is not the case however. We also have multiple possible pasts, and in fact they’re not just possibilities but are real. Each future universe with this one as a past is real. Each past universe with this one as a future is equally real (that’s the mwi view of Schrödinger’s cat). Because of the arrow of entropy there will be more of the former than the latter.
Hope that helps..