First, a team making a big change in the middle of the season rarely gets benefits this season.
For PHI, it's different, they have all to win to a trade, and nothing to loose compared to now. Any package would make them better, but that's why they can wait to get the best available, even if in the end it may be less than what they thought.
Second, it's easy to see that Simmons fits best with a shooting C, like better fitting with MIN than ATL. For ATL, it would certainly mean trading Capela, because as of now, the summer rumors seem to match reality here :Schlenk not wanting to offer a lot because in this roster, Simmons would be use as swiss knife, filling any situation where is a better matchup than another player, and this differently every game. But is it possible to have a 40M guy doing it ? In some years with a way higher Salary Cap, maybe, but now...
We have two C that don't shoot 3pts, so we need a lot of shooter around, or all the pressure will be on Trae.
If we do a trade of Bogdanovic+Huerter+Reddish as some may think, there are more chance that ATL would not be better than now, while PHI would 100% be better as it's three players are not stars but can shoot 3pts, run an offense (Bogdanovic), create his shot (Huerter) and potential to be a very good wing defender in Reddish.
And then, imagine if they face in playoffs... while in the hell would ATL do such a trade ?
I also don't forget that Simmons is a bit overrated on defense, in the sense that he was not a master on defense these 4 years he played. And he didn't stop Trae last season as everybody was thinking.
His 1st season was perfect, team and individual defense, but after, I don't know if he got a Kardashian syndrome dating Jenner in his second season (at this moment, only Booker seems to not be touched by that
), but it ended with PHI being at -2pts On/Off court (+12.4 for Embiid...) with Simmons. His individual defense was also lower with direct opponent PER going from 13 to 17.
3rd season was the same as the 2nd.
And at least, last season, he was good again, but less than his first season : 18PER for direct opponents but +6.6 On/Off court, compared to 13PER for opponent and +7.7 On/Off in the 1st season.
More important, having a good season didn't make him having the same impact once in playoffs.