It's governed because the stock tires are only rated for that speed. If you get a tuner and raise speed limiter, don't go over the rated speed for your tires. I'm pretty sure these trucks are capable of 150 mph or so. My door seals start to blow out at high speed, kind of unnerving to suddenly hear sudden rush of wind noise. Another thing to think about, how safe is a giant brick at 150 mph? I've seen some pretty bad wrecks of old gasser cars (40Willys type cars) that seemed to become airplanes at that speed.
The closer I get to death by virtue of age, the less I want to die by virtue of foolishness. I wish all vehicles were governed lower... But, hey, re read my first sentence - I ain't nary whippersnapper!