In what way do cyclists require unique and expensive infrastructure that both isn't the consequence of interactions with cars and that wouldn't be covered by general taxation unless general taxation literally didn't pay for any infrastructure?

Maybe isolated recreational paved bike paths, needlessly expensive public lockup places, and smaller scale urban infrastructure to avoid accomodate only pedestrians and bikes in forward thinking places? If cars weren't so common, would cyclists require 8+ lane highways, or even relatively wide roads? Seems like we pay into a pool of infrastructure funding that is often already very expensive and that has little to do with cars, if they didn't exist we'd broadly be saving public money, both on direct and indirect costs such as pollution, deaths, traffic control devices, public policy, or accommodating the demands of everything but personal cars as necessary. They should be treated as an expensive luxury, which they should be, but in some cases they're a necessary burden that the poor should be releaved of.

If cars weren't default, EV or not, we'd all be like "who's going to pay for that!?"

Likewise with trains, we all pay for them with taxes, but the people who use them often pay directly for the continued operation in terms of what is not their personal obligation (maintenance, construction, staffing), usually a relatively marginal source of revenue, but it keeps it going. You pay for trains through general taxation, and you pay somehow for the continued operation of your personal vehicle, and so do bikes, but cars demand much more from external sources like trains do, and like trains, there's no free ride, unless you bike, which has relatively minimal external demands. You pay for the continuance of the operation of a uniquely burdensome private luxury, and it's not subsidizing anything.

Roads also open up some amount of significant economic commercial and personal opportunity, which should also be factored in, but also paid for like others. If it's a problematic amount, then you make different choices, and if that didn't balance out at a system level, we'd make different infrastructure choices.