Thats an interesting point id like to pick up. None of the pokemon in A+ and S are set up bait because they are offensive pokemon/sweepers with the only exception of Mega Venu. For a while now i have been under the impression that defensive mons are considered to be less viable in general compared to offensive mons. Mega Venu is the best mixed wall in the game and was demoted to A+, Hippowdon and Chansey are also extremly strong mixed walls and had to dwell in a pathetic B+ ranking for quite a while now.
Seeing it from the perspektive that the best pokemon in their respective aspect (ofc given that its a highly viable aspect) Venu should be S and Hippowdon and Chansey would deserve A+. Its mainly because offensive mons are overrated here while defensive mons are underrated in comparison. Pretty much every lightly specialy oriented wall/tank stops Thundurus-I dead in its tracks, yet its S, while against Chansey ur using the argument that some things can set up on it.
Imo it's mostly because:
-Just going by the ranking definitions: it's really easy to sweep through a "significant portion of the metagame." It's very hard to wall a significant portion of the metagame. There's just too much crazy powerful offensive threats.
-Hyper Offensive is easy to use. Deosharp teams are letting complete shitters get above 1800 rating. On the other hand, defensive teams have a much higher skill ceiling. Deosharp is literally the name of the game on the upper ladder but in SPL there's a good amount of balanced, defensive and even full stall teams. It's really easy to throw together Deo-D+Bisharp+Aegishlash+3 sweepers and be able to tear through everything, it's much harder to make a defensive team that can counter or check all the insane offensive threats.
-Gamefreak likes offense. This gen, defensive teams got were pretty much Mega Venusaur, Mega Scizor, Chesnaught, Sylveon, Gourgeist, and that's about it, unless you count Mega Aggron and Avalugg who are way too flawed to be used on most serious defensive teams. Meanwhile, we got Mega Kangaskhan whose absurd power eclipsed that of gen 4 Garchomp, Mega Lucario and Mega Gengar who were also absolutely insane, Mega Zards, Mega Pinsir, Aegislash, Mega T-Tar, Mega Mawile, and a bunch of other threats that defensive teams can struggle to handle without niche stuff.
Offensive threats are just outpacing defensive threats, is all.
So that's probably why defensive mons are being "underrated." You just can't wall a significant portion of the metagame, but it's not hard to sweep it. HO teams are just easier to use. And if you'll notice, every S-rank threat right now is a stupid powerful thing Gamefreak gave us
this gen, with the exception of Thundurus.