I think one contributing question I can add to the discussion is to ask Retreater if there have been any common causes to TPKs or anything in particular that causes a ton of pain. I would also love to see any recollection of combat specifics, if they can be remembered.
Let's see what I remember.
Of course, the specifics of combats are going to have spoilers. I'll try to not link them to actual adventures so maybe it doesn't give away all the surprises.
(I will also add that at least a few of these are because of errors I made as a GM - but to be fair, the rules and guidelines aren't presented terribly clear. And one misstep can make a TPK.)
The first TPK was with an extreme encounter - a solo baddy (a bargeist at level 2 or something).
So he's in the center of a room. You can't pass into the rest of the dungeon without dealing with him. The group went in without a lot of preparation, being new to the system, new to the party configuration, and not having had tactic training like "you need to demoralize, trip, etc. to have a chance against an enemy several levels above you."
Once the unreasonably high checks to roleplay past the encounter obviously failed, his insane crit damage dropped the warrior-types almost instantly. He was getting - on average - two crits a turn (even with his multiattack penalty) because his attack bonus as a higher level was just so much better than the party's defenses.
The next TPK happened a couple sessions later. This was due to some lightning-dragons. Apparently I misread their abilities to do something like "draconic fury" and wasn't applying the multiattack penalty at the right stage in the attack. (Some creatures let you apply multiattack penalty
after all the attacks are made, but these specific ones did not.) We had a TPK, but after the error was brought to my attention, we "rewound" the fight and did it again. [So much for campaign continuity and immersion, but I guess it's better than two TPKs in two sessions.]
The next TPK happened the next session. The party was going through a temple and opened a door that triggered a magical hazard. The saves were already high enough that it was regularly doing critical damage to them, blinding them and causing some big damage. The door that they opened led into a room with cultists, so they obviously took advantage of the situation and started attacking the characters - since the module specifically said the cultists couldn't be harmed by the magical trap. (This was proof that the system can't handle two encounters being added together.)
The next TPK happened the next session. It was another solo baddy - like the bargeist. This time it was a hellhound. His DCs were so high the party was regularly crit failing their saves against his fire breath.
I think in each case it was possible that the party could've left a character or two to die and not stayed to fight in an attempt to save them. But in my experience, players will rather die as a team than run away and leave another character to die.
And yes, the party "would've/should've/could've" performed more tactically, identifying creatures, demoralizing, tripping, getting into perfect positioning for every encounter. But I'd like you to consider the composition of my group:
1) My brother-in-law, just there for the fun and to play with his family. You pretty much had to make his character for him and give him suggestions of what to do.
2) My wife, though enjoying tactics, doesn't spend considerable time out of the game reading message boards, watching tutorials on YouTube, etc.
3) My best friend, a casual player, who just wants to see what PF2 is all about. He frequently confuses editions (PF1, 5e, PF2).
4) Two players who did read up and study the game but wanted to test out a lot of options and try some unique builds just to put PF2 through the full playtest.
Few of them would be regular posters on here or care enough about a hobby like TTRPGs to devote hours every day studying the meta behind a game system. Paizo's design is clear that they don't care about these types of casual or new players. If they did - it wouldn't be so brutal.
When people suggest perfect tactics to have a thrilling, winnable game, they are excluding probably the majority of gamers and the potential audience.