winshares are pretty meaningless stat unless you think Jordan Bell is significantly better than Tatum. They also generally require a team to win and most of the time top tier rookies aren't on winning teams.
It's true that most top tier rookies aren't on good teams, but a big part of that is top tier rookies often don't help teams win games.
I don't agree that WinShares are a meaningless stat. I think they do a pretty good job of capturing which players contribute to winning as opposed to those generating empty numbers.
There are flaws to the stat, like any stat, and it does overrate big men (especially bench big men on good teams, e.g. Jordan Bell as you point out). But I think it's fairly spot on with regard to Tatum being an outlier. Very few young players, even those drafted very high, are as good defensively, as efficient offensively, and generally as poised as Tatum.
You could look at Jaylen Brown or Marcus Smart, the Celts' two most recent high draft picks prior to Tatum, as good examples of that.