When you can’t get what you want, you teach yourself to want what you can get and then preach to everyone else that they should want it too. This applies to many other things too beyond just cars.
It’s not that the criticism of private cars isn’t valid, but not having one because you can’t afford it isn’t virtuous. It’s only virtuous when you could easily have one but choose not to.
I also know fewer people who complain about cars while not being able to afford them. Once you’re out of college/university age, it’s pretty rare to not be able to afford a car at all - it might be a bit difficult, but certainly possible. Anyway, if you care about the right thing being done (i.e. fewer people using cars, better public transport, better bicycle lanes), why does it even matter if people advocate for it for the wrong reasons?