Not so much that, more a case of 1mm^2 is the smallest T&E used and anything smaller would start to get a bit fragile, not to mention the resistance would creep up - hence more brightness variation and longer disconnect times in the event of a fault.
Also there is a (not often used) standard lighting circuit protected with a 10A MCB, and that is somewhat closer to the 1mm^2 11.5A "in conduit" rating for the cable.
Indeed... A reflection of reality that in many properties, the only electrical provision installed was for lighting. Sockets etc came later.
I suppose you could install a 0.75mm^2 T&E if there were such a thing, but I expect in most cases the savings would be marginal.