The difference is that failing to inspect your car before you drive might put you on the side of the road while you wait for a tow truck. Failing to preflight your airplane will kill you if you do it habitually. This is doubly true when your airplane folds up and you unfold it right before flight.
You absolutely need a flight plan. You do not need to file a flight plan, but you still need a plan. You need to know where you're going, how far it is, what the weather is like, what the weather is going to be like over the course of your trip, what your alternates are, what NOTAMs are in effect, etc. This step is, again, not optional if you enjoy living.
The price premium of these roadable airplanes will pay for decades of hangar rent and car rental, no exaggeration, so I don't see either of those as a disadvantage.
Also you don't need a flight plan, that is completely optional (it is essentially a SAR service if you crash). With modern avionics you can dynamically recalculate flight paths on the fly in the cockpit.
You really don't need clearances either, unless flying into class B airspace (which you don't really want to do that with a small plane anyways). You can fly across the country without ever telling anyone.
Also having separate planes and cars has real disadvantages. What about when you arrive at the destination? You always have to rent a car. Not only that you have to store the plane which can be quite expensive. There are many advantages to this.