And now an important public service announcement.
Just because the game recommends a team of Pokemon to battle with does not mean that you need to use them.
I understand that this game has brought out the noobiest of noobs when it comes to Pokemon and their typing but common sense must still prevail, ESPECIALLY when we have been raiding for this long. Which is why the picture below is unacceptable. This thing didn't even have solar beam and we still barely got it down with 10 people, (the team also included a Skarmory at one point and I was too angry to look at the rest.)
Also by now if you must utter the phrases "What's this thing again?", "What's good against this thing?", and "Blissey lasts forever." Please stop playing.

Answers
It might be more egregious than you think. I haven't seen the autoselect recommend a Blissey in a long time.
The more important PSA is:
It is okay if your six attackers faint. It is okay if every players' team faints. As long as there is time remaining on your fight, you can rejoin and keep going. Your personal damage contribution bonus is no longer reset when you rejoin.
Also everybody's health was reset last night, so they likely went out of their way to put Blissey in. Only other thing is if they previously had battled Solar Beam that day and it killed a lot of theirs. Maybe they didn't go back to try to health anything b/c this group was big enough (and it was, no matter who did what)
Just a week ago I caught my first legendary (second legendary raid)... Suicune... and it didn't just make my day, it made my week! I'm almost level 39, have been playing since Oct. last year, but hadn't had a chance to get any legendaries before (due to being a rural player; the legendary raids were in the city).
But you better believe I knew my counters ;)
I’m level 38 and still ask myself from time to time "what’s good against this". But I’ve only been playing for about 3.5 months (which is what your "by now" comment means to me) so perhaps the complete type match-up chart as well as the list of what type all 250+ Pokémon are hasn’t had time to become an instinctive part of my being yet.
But when I’m picking Pokémon for a raid, if I’m not sure what to pick I’ll switch over to here and look it up.
So tell me again why I should stop playing please?
I find nowadays I'm just happy to run in a group that doesn't know what the good counters are, since that means I get a chance for 3 damage balls which is usually impossible due group sizes. If someone asks, I'll say out loud what they should be using, but if nobody asks I'll let them shoot themselves in the leg if that's what they want to do.
I have to say I have not seen a Blissey in a raid since early autumn. Maybe information circulates better in my area than yours.