That's because they're the only thing that they make well.
Now, many, many, MANY people who drive big pickups or Excursions, Suburbans etc. don't need them and they are complete pricks as far as I'm concerned. They're the issue, not the auto manufacturers.
The average mpg thing is stupid. This is a consumer issue.
Got a lot of kids or need some moving around to do? Get a minivan. They're cheaper, more fuel efficient, safer and typically have larger interiors.
Going to the mountain, or driving in adverse weather conditions? There is an array of AWD cars available that can handle their business far better than most trucks or SUV's in that category.
Basically, the only people who should be allowed to buy SUV's are those who tow things on a regular basis.
If there were a system to allow people who actually need giant 4x4 whatevers to buy them normally, while taxing those that are buying one because they're insecure about the size of their gentleman sausage, I'd institute it tomorrow.