Splits happen forward in time for the same reason a glass which has fallen and smashed on the floor doesn't spring back up and spontaneously reassemble itself. And these "universes" are really just isolated amplitude blobs in the total, timeless, wavefunction: they aren't created; rather any amplitude blob roughly factorizing as a "world" will eventually decohere into several smaller amplitude blobs also factorizing as "worlds" which as the wavefunction further evolves with time do not interact (i.e. they interact about as often as that glass reassembles).