I don't think people would pay the true cost even if they considered the 'cost' of driving for such a long time, petrol, extra wear and tear, having to potentially drive on unfamiliar roads, hotel stays etc.
We used to use the Harwich to Esbjerg ferry as it was a nice way to save a lot of driving, have a bit of a break, but it wasn't cheap. And that got axed (albeit for a few different reasons that all came together to make it unviable).
When going on a 2-3 week holiday, it's nice to pack the car up with stuff, not pack it all up in suitcases, haul it onto a plane, then off, then to a hire car and the same when coming back.
A motorail service though parts of Europe, perhaps for really long journeys west to east and vice versa, where the costs in the first paragraph mount up more, could have a market - but, again, I don't think people would accept the cost even if it was cheaper overall.