For two reasons:

1) At channel creation time, the peers necessarily are not joined to the channel (since the channel doesn't exist yet) and therefore cannot act as anchors.  So, from a workflow perspective, setting anchor peers before the channel exists would create a transitively incorrect configuration.

2) Generally, Org1 should not be able to set anchor peers for Org2, Org3, etc.  It is possible (though I would strongly recommend against it), to craft a channel creation transaction which sets anchor peers for all orgs, but then you would require admin signatures from all organizations in the channel, and not simply the creator.

Thanks,
~Jason