What's so bad in raising speed limit to 85 mph when most people already drive that speed?

most interstate traffic goes at 70 to 75 at minimum. Typical average speed is around 80. Where I live the highway limit is 55 and 99% people ignore that 55 limit. Anyone who drives 55 is left in the dust. I myself find it hard to go below 75, because that's the speed at which 85% of the motorists go. Going below the average flow speed, endangers your life. You can get rear-ended. Speed survey in my area shows 85th percentile speed is 80 but limit is 55. Since most people already go around 80, why is it bad to raise limit to 85 in Texas?