CUBluejays, you are right about lots of high school pitchers not hitting...the #1 problem virtually all amateurs would have vs MLB pitchers is the COMBINATION of breaking stuff and FBs.
Some amateurs can time 90-something, but can't begin to handle the differences in pitches thrown from the same arm slot to different quadrants...and MLB pitchers can "smell" what amateur etc. hitters are sitting on. A lot of very good high school and college hitters can't handle good breaking stuff, which is why they can't advance to pro ball at any level.
Adding changing of speeds wouldn't really be necessary for MLB pitchers to wipe out amateurs.
If given 100 ABs vs a MLB pitcher in season, an amateur might get lucky now and then, but it would take them one helluva lot of reps to even get to that point!
Hitting a baseball thrown by an elite pitcher (as all MLB pitchers are) is REALLY REALLY, hard!