It can entirely depend on the deals you get; I was able to rent a Tahoe (largish but not huge SUV) and drive it across the country and back for about a grand; unlimited miles; and I've done similar with a minivan. You get a brand new vehicle that you can put miles on and it's unlikely to fail you.
True, deals can sometimes be found but that's more work and worry when you should be relaxing on vacation. Combine that with the other drawbacks (dealing with pickup/return, worrying about scratches/damage, not able to have customizations like roof racks, going offroad or through mountains with crap rental tires, etc) and it's easy to see why renting is not more popular.
I don't think people arguing this have ever actually run the numbers and realize just how expensive rentals get. Every year my wife and I plan a week long road trip. If I price out this years trip from the midwest to Utah in a full size suv or truck (need 4wd and space for climbing/camping gear):
Base rental: $970
mileage fees: $350 (1400 miles at 25 cents a mile)
Tow fees: $180 (optional, but I like to bring my small teardrop so $20 a day for the tow package fees)
We're now close to $1500. At this price I can buy a truck, title it, insure it, and then sell it at a slight loss when I get home and still come out ahead of the rental. In just one trip.
*edit - formatting