Single-occupancy vehicle users don't sustain the current highway system.
If tomorrow, freight and delivery trucks somehow ceased to use roads and highways -- we would be screaming about how SOVs don't economically sustain the highways. Driving to/from work is a luxury, and we've made it into an unsustainable necessity.
I'm saying as bad as you think public transit is in terms of economic viability, SOV use is that much worse.
Edit: Let's not forget that lots of transit in the U.S. used to be private too. And they went bankrupt because governments started spending a ton of money building roads to compete with them. This is still going on now, except government is running both now (See: Translink).