There’s literally no reason that people who can’t afford transit need to pay for it, but at the same time, there’s literally no reason that people who can shouldn’t.It'd be weird to say health care shouldn't be free because those who can pay for it should. I'm not sure what the difference is here. There are many good reasons for free healthcare, it's harder to take away when it's for everyone, means testing is expensive, and people who can pay already do that with taxes.
Most transit systems the majority of their costs are covered by the government and not fares. Fares definitely do not cover the cost of expansion. The exceptions like Hong Kong and Tokyo might be profitable but it's because of real estate and their stations, not fares.
For transit enforcement is also expensive. Beyond turnstiles, tickets or cards, police have to be paid to stand around to catch inevitable turntable jumpers, and this is not a good thing for multiple reasons. I also genuinely don't know how tap to pay with your credit card works. Maybe transit systems are able to negotiate with debit and credit companies, because normally high volume small transactions don't make sense to do. You lose too much money to the credit card because of the flat fee; this is why you may see bakeries require a 3 or $5 minimum to use a card. This is also why you don't see a competitor to patreon for monthly small donations. Even patreon can't really make reoccurring high volume small transaction payments work.
having fares makes sense as a way of managing capacity and travel demand.I think this is the strongest argument, but I think most places in the US and Canada we're not in a situation where too much demand is a problem yet, and there is also plenty of room for expansion. Inducing demand in transit's probably not a bad thing.
Also, Luxembourg managed to make their transit free. Seems to be doing well. Maybe it's not, I haven't looked super deep into it, but the reasoning they use makes sense to me. Fares only covered 10% the cost of transit there anyway. I'd be interested if you knew more about this, for sure.
luxtoday.lu/en/knowledge/why-public-transport-is-free-in-luxembourg
(Fare systems that have "zones" also suck to navigate honestly, though that's not an inherent quality of fares.)