You said it yourself - you literally can't use public transport without either standing or sitting on their chairs. It's a physical limitation. But it's entirely possible to use public transport without having ads shoved into your face. It's even the default! If people didn't put up ads in/around public transport, you could use public transport without seeing any.
They could simply put the chairs you like into their cars though, there's nothing stopping them from doing that. They just choose not to, much like they choose to put ads in the windows and on the door - for economical reasons.
There's obviously one issue stopping them: there are far more people using public transportation than there are chairs in there, so it's once again physically impossible to accommodate everyones tastes. This is not the case with advertising.
