i've wondered this myself. i often hear people say stuff like "people with real adhd can't get high from stims", but i suspect that some of this is just that people want to distance their legitimate use from those tweakers over there.
if there's any truth to it, i'd bet it comes down to the way the drug is used. when you take an extended release formulation (or instant release every n hours) regularly, you tend to have relatively stable blood concentrations of the drug (ie, you don't get "high"). recreational users, and also probably people who self medicate, will end up with peakier blood levels, which results in the perception of being "high". the concentration level over time can be an extremely important factor in how the drug ultimately affects you.