What's harder for me to get a hold on is the people who say some things - transportation, education, healthcare, housing - should be public and everything else should be private. I'd like an argument for why those things and not everything. Why buses and taxis? Why trains but not planes? What about boats? Why people but not goods?
I guess I can take "politics" as an answer but that is a bit disappointing.
Profit has to come from somewhere. In some cases, it can come from neglecting to maintain the infrastructure that the investors (usually the taxpayers) paid for to put you in business. Once the system collapses under the neglect, everyone is held hostage because the system is vital and then taxpayers are forced again at gunpoint to bail out the system. In the mean time, the profits of the mismanagement have been taken and are effectively unrecoverable.
The "politics" that comes in here is when people refuse to be robbed in this way and, instead, insist in a return on their investment (in the form of a functioning public utility). This is a consequence of democracy (fortunate or unfortunate, depending on your perspective).
What things should be nationalised and what privatised has been extensively argued. We also have a lot of real world examples of what happens when you nationalise companies. The UK nationalised all its heavy industry and most of its light industry after WW2. It didn't go well. French nationalisation of its finance sector in the 70s didn't work great either. The things that are commonly nationalised are those that aren't huge, enormous, obvious, fast fuckups. Some countries do this better, some worse. But mostly it's kludges based on experience, not principles.