Speaking from experience,I disagree!
We have been on many long car trips (as in between 8 and 16hrs) with our daughter who is now 5.
Before she could speak she would just sit and stare wide-eyed at the countryside flashing past, now we talk, sing, take-turns drawing on an etch-a-sketch type board, play guessing games, read , etc...
She has also learnt to spend time occupying herself, and has also learnt that at certain times she just needs to shut up and sit still because daddy is concentrating on traffic and finding the way.
OK, it probably varies from kid to kid, but don't say it's impossible!