My understanding of ebgp-multihop is that it increases the TTL so that we can reach for example a loopback interface whereby otherwise the packet would be dropped at the physical interface.
Q: When configuring iBGP with loopback addresses why is an equivalent of ebgp-multihop not required ?
Because it is a requirement for EBGP but not IBGP. Basically IBGP uses the internal IGP ie. EIGRP/OSPF to have reachability to other IBGP peers or there is a fulll IBGP mesh, one or the other, but EBGP is between 2 separate AS's so there is no IGP being run between the 2.