Why is it required that iBGP peers always form full mesh?

eBGP uses the AS_PATH as loop avoidance mechanism since iBGP router doesn't change AS_PATH attribute while advertising routes to another iBGP peer we can't use it as loop avoidance in iBGP. To avoid loop iBGP peer doesn't advertised routes learned from another iBGP peer. So in order for an iBGP peers to receive all routes need to establish BGP session with all other peers hence full mesh.