It's actually not that surprising that a player of Sinner or FAA's level would drop out. The Next Gen tournament is basically an exhibition. The tournament does not provide any points towards competitors' ATP Rankings. If you win, the ATP does not recognize it as an official ATP Tour tournament triumph, although matches do count towards the season's official win-loss record.
For example, if you look at Sinners ATP bio, it says he has won 5 ATP titles... not 6. That's because the ATP doesn't recognize his Next Gen win in 2019 as a title.
The Next Gen Finals also uses funky experimental things, like best of 5 Fast Four scoring with no ad. They were also the first tournament to do completely electronic line calls with no judges. I'm not sure what new things they are trying this year.
The primary reasons to play the Next Gen Finals are for money and notoriety/exposure. In 2019, Jannik was an relatively unknown player that had spent most of 2019 ranked outside of the top 200, only having won a handful of ATP Tour level matches, but was on the rise with a recent Antwerp semifinal that year. Going into the tournament, he actually wasn't ranked high enough to be in the top 8 of the Next Gen standings, but got the Italian wild card entry. However, as we all know, he thumped the field and destroyed poor De Minaur in the championship match. He took home over $400,000 in prize money, which was massive for him at the time, and was suddenly on the radar of everyone that was looking to see who the next ATP star might be. Sinner was then able to parlay the newfound fame into more endorsement deals and wildcards into additional ATP events, which helped him in his ascention to where he is now.
For many of the guys in the current Next Gen draw, especially the two from Argentina, this is an opportunity to gain some much needed cash and possibly break out with some big wins. I've seen everybody but Cerundolo and Baez play, so it's a chance for those guys to get some exposure and see if they can beat a more famous player like Alcaraz or Korda. It's too bad that Brooksby got injured because if he was in, 3 US players would have been in the draw, and we all know that the US is desperate for a champion, or at least a consistant top 10 player. An win by an American could help that player gain some traction in the media, I suppose.
For Sinner and FAA, already ranked #9 and #11 in the ATP rankings, the points in Stockholm are more important for them heading into 2022. Sinner has a chance to end the year with his 5th ATP title, and FAA with his first (which is crazy to think about how successful he's been without winning a title yet).
