Nyashes

You're right, PL-3 summons can be relevant in combat and worth the action tax if you pick correctly (skunks and giant skunks come to mind as powerful outliers). PL-4 and PL-5 aren't in most cases, that put the "summon cutoff" at around level 7, even if in my experience there aren't that many good options competing for the "haste" spell slot

beats me. Honestly, I'd say someone probably started saying it was amazing and then everyone started repeating it, but the argument of "look at this free flank" or "wow, such body block" are the only tangible ones I've ever seen coming from there, and the body block one is absolutely risible when Illusory Object exists and does a better job, without sustain, for one less action, and for a static 1st level slot instead of a max level one, the flank, on the other hand, is everything but free and likely not that good if the melees are positioning themselves correctly

Yup, summoning being, let's be real, a pretty bad combat option is likely by design, and probably not a bad design decision if anything, they did make a class centered around the concept if that's your thing (summoner), with a reskinned animal companion archetype for the "necromancer" trope, while ensuring people couldn't cast "conjure barbarian army" and make a single fight last 4h during which it's your turn for 80% of it.

I don't think Paizo wants summoning spells to be used, but a lot of people online will swear it's a good option. It really isn't and clearly wasn't meant to be, and that's OK. I usually consider that PL-2 to PL-3 is guaranteed to be weaker than a single character action for action (even including the 1 sustain for 2 actions) and would have been a relatively "safe" tuning point to use, PL-5 is almost doubling the safety margin, that plus the fact Paizo happily erratas outstanding summon options that would be viable despite the level disparity is a clear indication they wanted to make real sure this wouldn't be a strategy

If what you've seen comes from r/pathfinder2e on Reddit, it's likely because the place is a borderline echo chamber nowadays, and will happily swear that everything short of dumping your key stat is viable regardless of whether it actually is or not.

You'll have a hard time finding any mention of something in the system being "not as good as the rest", or, god forbid, the system having specific shortcomings (you might see "the game has shortcomings" but specific mentions will be absent).

I really invite you to check multiple posts and look intently for those less positive outlooks, you won't find many and those you will find are likely to be at the bottom, on the other hand, you'll see a ton of echoed opinions about the absolute superiority of pathfinder 2 math, game design, class balance, etc... either over other popular system or in absolute.

I don't think the moderation team there is complicit, but it absolutely has let it go to shit over the years

TL;DR: to answer more specifically to "What is it that makes people really like summoning in general?" pf2e Redditors like everything about the system whether it's good or not

yeah, there might always be that summon in that case that has this ability doing the right thing, but you already have spells as a toolbox, and it's rare you can't do whatever you wanted your minion to do better yourself. It goes without saying that counting on anything with an accuracy is basically moot with the 10 points to hit vs defense differential, and very few summonable creature have good guaranteed support (a lot of them were either moved to uncommon or errata'd, like the Bone Croupier recently). To me it's rather clear that Paizo doesn't want us to use those strong accuracy-less effect through creatures and we're only left with a few non-disruptive scraps

I'm however completely unconvinced about soaking damage, for that to happen it needs to be threatening, and, excluding some specific situations as mentioned above, they aren't, and are as likely to hit as a player is to crit, which, on PL+1 and up, is probably only on nat 20s. If your GM sinks an attack into that, he's saving you from yourself by removing your sustain tax. In fact, Illusory Object is a better body block since it's way waaayyy larger, is a 2nd level spell at most, doesn't require sustain and also requires at least an action to disbelieve which uses your spell DC as a "AC" instead of the AC of a PL-5 creature.