Economics and big business. As Richard has mentioned, it costs a lot to make corn alchohol right now.
A better question is why the manufacturers keep selling us cars/trucks that get mediocre mileage. The automobile has been around over 100 years. All the manufacturers are on a new/recent engine designs that replaced older less efficient ones.
So, how come we still get mediocre mileage? They burn cleaner, and have more HP, but they do not get significantly better fuel economy compared to the previos generation of engines. Heck, my Dad's '63 Chevy Super Sport Impala with a HO 327 would get mid 20's on the highway(IF he kept his foot out of the secondaries on that Holley). What do full size sedan/coupes get now?
It burns my hide, 'cause it's all big business. First, we use up non-replaceable fuel source(dino-oil). Second, it costs us a lot at the pump. Third, the autos use a lot of it.