I'll trade my starting shortstop for your number one draft pick.
The irony is that NFL and NBA draft picks are worth a lot more and yet those two professional leagues allow trading, including to move up or down in the draft itself. Football and basketball draft players primarily from colleges and their draftees are much more finished products and therefore more likely to match expectations.
The Major Baseball League (MBL) must send its draftees into its minor league system and patiently wait for them to make progress going from rookie league to A, AA, AAA. Baseball players matching expectations, even for those who played in college, is much less certain.
So why not let teams trade draft picks?