They are talented, they just aren't that experienced ... yet.
Everytime one steps up a level --- and the difference between HS and college ball is a BIG step --- things speed up. Teammates and opposing players can do things and do do things that you have rarely seen before. A pass that was open in high school or even at AAU level for a second isn't there that long. Shots you used to take over players end up back in your face.
Big, tall, athletic guys are going to get better and better and better.