Because it would have been economically foolish to have done so. IIRC, Edey was listed during his Junior season in assorted mock drafts to be late a 1st round to 2nd round draftee, with more favoring the latter. By staying at Purdue and becoming a 2-time NPOY and only the second in around 35 years, he moved into around mid-lottery as a draftee. At the same time, he was reported to have made money at Purdue that was not substantially less than a 2nd rounder.
Being in the lottery as a draftee is far superior to being a 2nd rounder. That is true for dollars in the contract, length of contract and guarantees of contract. Plus, he got some national ad endorsements. On top of that he further developed his game. Staying for his Senior year was a wise decision on his part. I am astounded to read that you do not recognize that. But given your other analyses, I shouldn't be surprised.
As for JHS, according to ESPN.COM, he has played only 2 games this season for the Lakers and averaged 2 ppg. His career stat is 1.7 ppg over 23 games. One can argue that he was not ready and came out too early before he was ready enough. You seem to have it backwards.