synths/samplers

What makes Synthesizers and sample based instruments sound " thin " up in the high registers ? For really low notes I think sometimes they are better than real instruments if you have a decent sized speaker . Not so for for synths and especially samplers . I wonder if the sampling rates wen't up really high would that help ? I also wonder that when a person tunes a real instrument that something happens when they go higher . My synths sound more out of tune up high too to my ears . It would be an interesting project to build a synth or sampler with a really high sampling rate and then to tune every note and see if it would then compare to a "real" instrument . With "real" instruments the "string tension" or "embouchure" pressure gets much higher is you get really high which changes the sound and makes it "scream" . How would you imitate that with a synthesizer ?