IMHO, its a mental thing. I mean think about it. Most of use hit of a tee even on a par 3, yes, the tee maybe level with the ground, nevertheless its still on a tee. You know the distance, you can see the flag, meaning depth perception from a 150 yard tee box is easier mentally then say, standing in a fairway looking at the green, looking for a yardage marker (if you don't have a GPS). I could go on but I feel like I have soapboxed it enough.
*Disclaimer: No animals were hurt in the composition of this rant.