What stops someone from buying their car in a neighboring country?
The tax does decay for second hand cars, so there's a big market for foreign newtimers.
Huh, you can't even buy in US a decent new European car if someone decided not to sell the model through the dealership. Wait 25 years and only then you can import and register it.
They'd still be taxed, unless they lived in the foreign EU country for 12 months and bought the car and owned it for 6 months.
I suppose the EU is a customs union, but "some restrictions apply".