...because it's not the NFL. They are going up against completely different offensive schemes. The read option is flourishing in college due to speedster RB/WRs and athletic QBs. But to say they don't disguise anything is going a little far. They certainly do disguise, DCs don't sit around scratching their butts all week. They have a job to do and that is coming up with weekly schemes and disguises, just like the NFL.
But again, this was LSU. They are not "vanilla" on defense. Some of the defenses Prescott had to face in his college career are the closest to the NFL-level you'll face in college and is most likely the reason he looks so NFL ready.