Because if we judge a transport system based mainly on private motor vehicles by the same standards we judge every other form of transport, it would show how very ridiculous it is. It starts in the way spending millions per metre of motorways is handwaved as "investment", but spending on railways is "subsidy", and has to show a "return": if motorways had to generate a "return" they'd never be built.
In the same way a council that needs an excuse not to build a cycleway will demand an "environmental assessment" that that has standards any road would fail.