Doesn’t it make sense for public transport to be free to use and completely tax funded?

Public transport often runs at a loss anyways. It would encourage more people to use it instead of cars and probably to go out more in general and do more things. It would be especially helpful for the poor. It would save the time and efforts of dealing with tickets, cards and policing them. So far I've only heard of Luxembourg doing this. Why isn't it more common? What are the downsides?