Once you realize the sheer amount of commerce and business that is enabled by roads you see that they pay for themselves several times over.
That's even easier to measure because these gains are localized.
Specifically, though, viewing roads or metro as a "loss leader" is probably an inappropriate analysis.
Also some studies have calculated the impact on health in the society and calculated a cost to the society per km of driving a car and the money saved when doing the same km walking or with a bicycle.
If you tried to do that for merely the operating costs of many transit systems, they’d enter a death spiral.
It may help to either tax them directly, or indirectly via diesel. Yes, costs pass to consumers, but it would also encourage more done via ship and train, I feel. Even that would be a huge help to clearing up traffic and lowering infra spend.
The lack of bus drivers simply comes down to an unwillingness to pay more. It's the exact same problem with teachers, where the wage is extremely low but they would rather just have a shortage rather than paying more.