Tatum is not a good enough dribbler and passer to do that and he also lacks the explosiveness/strength of Simmons.
Having said this, in other areas of the game he is much better than Simmons.
Different players.
Besides shooting, what areas are Tatum better at?
I feel like shooting is a pretty huge area of the game. Tatum is also much less turnover and foul-prone, which is pretty impressive considering age and Simmons having an extra year in an NBA system.
Tatum is 3.5 fouls per 100 possessions. Simmons is 3.6 fouls per 100 possessions. I'd say that is about even. As for turnovers, Tatum has the ball a lot less than Simmons does. In fact Tatum's AST% is actually a lower number than his TOV%, while Simmons' is nearly double. So Simmons has a 2 to 1 assist to turnover rate, while Tatum's is negative. Simmons rebounds, assists, steals, and blocks at a higher rate (though blocks are fairly close). Tatum is a much better shooter from outside of 10' (which includes the line), but that really is all he is better at than Simmons.
I think this boils a huge part of the game down to one thing in kind of a simplistic way.
I mean, Avery Bradley is a better shooter than Simmons too, right? Does that mean that Tatum and Bradley are similar in that that's "all they are better at than Simmons"?
Tatum is a better shooter from 3. He is better from the line. He is better from mid-range. Those are three separate areas of the game. These distinctions matter: in a lot of other discussions we focus on specifics like this when comparing players.
He's also better at catch-and-shoot scoring, and pick-and-roll scoring, and in ISO scoring. I'd say the disparity is lower when you talk about off-the-ball offense (like cuts and alley-oops). And Simmons is very good at drawing fouls, maybe about as good as Tatum.
Again, in other discussions we will often differentiate those things (when, say, comparing Harden to Steph).
The bottom line is that scoring is a huge part of the game, and Tatum is dramatically better in a lot of different areas. He's already a diverse scorer who can get his own offense or let it come in the flow of a play, from literally anywhere inside 25 feet.
I'm not saying Tatum is better, to be clear. Just that we shouldn't understate how big the difference is when it comes to scoring.