Because it really doesn't work -- one payment channel works, but that only allows balance reconciliation between two parties (which isn't terribly useful). Lightning aims to construct a routing network from individual channels and there is where things go awry. The channels on each side of a node must be kept in balance. That turns out to be non-trivial to achieve. Today there is no "visa scale" traffic on Lightning. So what you say is both questionable from a theoretical perspective, and certainly not proven in practice yet.