I'll pass on your patronising tone, since I am feeling calm enough, thank you very much.
You seem to have missed a few important points here, though:
Yes, it is just a game but since its a game you're playing with other people a certain amount of consideration for them and therefor a certain amount of effort should be expected. I'm not sure if you actually meant that but "0 effort" isnt even okay for normal dungeons - since 0 effort would kinda be just standing at the entrance and moving your character every couple of minutes to avoid the auto-kick.
If we put effort on a scale from 0-10 with 10 being Savage/Ultimate level and 0 standing at a dungeon entrance, I'd expect something between 5 and 7 from your average DF-person in an expert and something from 4 to 6 during leveling runs (I'll drop to a 3 for a totally new person in the first few dungeons). That includes things like: knowing when to AoE and when to single target and how to make use of your full toolkit. A healer can probably heal someone through a dungeon by only spamming their very first healing spell, the group will still finish the dungeon, but that healer wasnt doing a level-appropriate job in most cases.
Vote kicking or leaving arent always feasible options and we all know how well even polite and friendly advice works with a lot of people. So often the best option for people is to grumble a little, finish the run and then rant about it somewhere, which might lead to a wrong impression of how much they actually care about numbers.
Thats why I'm assuming that the OP is blowing this way out of proportion - maybe I'm reading undertones here that arent there and if so, I apologise. But the way this sounded to me is if this is a widespread problem, affeting a majority of the population instead of just a niche-clientel. Pair this with the anti-parser-stance you'll find quite often here and you'll end up with the impression that this isnt a genuie question out of curiosity, but a stealthy anti-parser-thread. Again, if thats only me seeing things that arent there, I apologise. (I still feel that a term like "depressed", thats actually describing a mental illness, shouldnt be used so lighthearted)
I'll also repeat that in my opinion the problem often arent the numbers but the reason for the numbers, aka underperforming players, that cause the frustration - and that I've seen far more severly underperforming people in my time with the game than people rubbing those numbers in someones face.
Which kinda brings me to my third point, which might be a real answer to the question at hand - if it was made in the good faith of trying to understand why someones enjoying something you personally arent: As pointed out correctly this is a game that we all play for fun. "Fun" can mean something different to each and everyone of us, though and achieving something thats "fun" to you can be paired with a certain hardship or even cause other people to not understand you.
Glamour for example is fun to many people. Personally I'll never understand those who dress their catgirls in super sexy outfits, but if thats what they enjoy, who am I to question that? I'll also never understand people who Fantasia every other week or spend hours with Jandelaine (seriously, there are better looking Elezen). But thats fine, if that what brings them joy in the end - even if its paired with the frustration that Au Ra horns are bound to their faces.
To me caring about your numbers above all else is kinda in the same court: For some people seeing those big numbers is whats most fun to them, so they'll pick a job they can achieve that with even if it might not be their favorite job to play - compromisse, we all make them (To refer back to my glamour-example: I ultimatly picked FemRoe as race, because I could achieve whats most important to me about character apperance - long hairstyles - with that race. Now I actually love everything about her, but I made the switch because of that one thing).
People look and care about their numbers because ultimatly thats what brings them joy and makes them enjoy the game, even if the process to get there is a painful one - and even if they only/mostly share the painful experiences they had on that way.
So, to actually answer the OPs question, I'll once again doubt the assumption that in the end (most) people are only getting cynical and depressed about seeing those numbers and instead get a substantial amount of their enjoyment from the game by pushing themself here. Its not something I personally enjoy and if you want an answer from someone who does, I'd advice to create a new thread thats phrased a bit differently (for example by not assuming that someone whos looking at their numbers and cares about them must suffer from a mental illness) and hear from people why they enjoy that so much if you cant wrap your head around that (which is totally fine, btw - not everyone needs to understand why people might enjoy something different from what they enjoy).