Here's why: His game, based upon what I've seen, will not translate well to the NBA... even if he were to gain muscle.
Shooting: I think Kelly Olynyk is a faster shooter and dribbler, and Kelly is thus far unable to get separation for a clean shot. All too often Kelly has a very nice pump fake, and where a normally fast player could make a one or two step move (or put up a shot very quickly w/o dribbling), Kelly slowly drives and the defenders are able to get back into position... leading to a Kelly pass. The same goes for Jerekbo. Bender's form does not look as good as Kelly's either, so simply taking more shots in the gym will not improve any of this.
Rebounding: His form is poor in this category. He fails to block out, and when he does he finds himself directly under the hoop or out-muscled by guys. Players in the NBA will kill him around the hoop in rebounding.
Speed: Players in the NBA will get out on a fast break before Bender can regroup. He's probably on the slow end of 7 footers, despite weighing some 40lbs less.
Mismatch: We like to think of him as a talented 7 footer who can create mismatches on the perimeter, but he's just as likely to create a mismatch against himself (bigger/stronger/faster players)
Production: As others have highlighted, he hasn't produced much yet. While the players in his league are talented, they don't appear to be nearly as fast or as strong as the players in the NBA.
He has some attributes I like, but a #3 pick in the draft should come with a hell of a lot more than what Bender is bringing to the table.