I have been in thousands of experts, normal mode raids, normal mode trials, etc. over the years and never once seen anyone get kicked or shamed for 'underperforming.' The amount of time you'd waste kicking someone and look for a replacement that -might- be better isn't worth it; that's what turning your favorite streaming service on and watching something when you see that single-targetting BLM in a pull of 9 mobs and know its going to be a slow run is for. The only time I ever see anyone get kicked is in harder content, where a person's dps can in fact prevent clearing said content, thus requiring you to remove weak links. Not to mention, trying to shame someone about their performance has just a likely chance of a GM dropping into your DMs if they decide to report you for harassment, so very few are going to comment on that basis alone over an expert of all things.
You can easily shave off 10+ minutes from the average DF run if you pull giant, sprint between pulls and know your rotation for bosses. 10 minutes isn't a small number, especially when you do it every day.
Lets look at this another way, through some hard, cold, uncaring, math.
One group pulls 3 mobs. You have 2x BLM that for some reason, only want to use single target. the other group has 2xBLM that are ready to unleash AOE and the group pulls 3 packs of 3 mobs, for 9 total.
The small pull group BLMs enter their fire phase, and use Fire 4 (300 per cast) x 3 = 900, for 1800 potency between them.
The other BLM group uses 3x Flare, for (260 (first target) + ((260*0.6 (Flare falloff potency)) * 8(number of remaining targets))*3) for 4524 potency, or 9048 potency total between the two of them.
The speedrun group, in the span of about 9s, has done almost 5x as much damage as the 2x solo target BLM group in the exact same time. To put this in perspective, the 2x AOE BLM group just did 45s worth of damage in 9s, meaning they saved 36s. For that single instance of a single pull. Magnify that over every big pull you can do in a dungeon and it adds up FAST. Add in Freeze, Thunder 4, etc and you get an entire single target rotation's worth of damage done in 2-3 GCDs. And that's just on the dps. Throw in comparisons like a healer not dpsing vs one chain-aoeing holy/gravity/Art of war, and a tank fully utilizing their aoe rotation instead of 1-2ing or god forbid, single targetting and the numbers just balloon like crazy. When giant pulling, AOE skills do ridiculous amounts of damage compared to single target. The comparison isn't even remotely fair for single target.
You may not care about tomestones. But an intensely far, FAR many more do, as saving up tomes to buy that shiny new piece to improve their character is fun. And Expert, for time spent vs reward, completely crushes everything else you could possibly do to earn tomes.
For the person just hopping on to do dailies, for the person who maybe only has a single hour of playtime a night to play, for every single person that may want to farm tomes in another way like maps but has no friends online to come join them, Expert is by far and large, the best option. So when they've seen the same dungeon for the 50th time, chances are they're not interested in taking their time strolling along and admiring the scenery, nor are they interested in a tank that seemingly wants to make the run slower for some arbitrary reason when gigapulling is ridiculously easy unless you're severely undergeared, they want to get their stuff done and get out.
It's an inevitability that people will find the path of least resistance and abuse it to finish their current objective so they can get back to other content they might find actually fun. Take a look at literally every step of the old relics - I remember when people spammed the Aetherochemical Facility dungeon over 30+ times for tomes for one Heavensward relic step. I can pretty much guarantee very few of them found it fun; but it was the fastest and most efficient, hence why it was by far and large the most popular method to deal with that step, despite being the pinnacle of an unfun grind.
MMOs are filled with busywork that most won't find fun. But people do it anyway because the shiny gear or upgrade on the other end does give satisfaction. If you don't care about tomestones and find fun in spamming WoD all day, all the power to you. But understand that people have wildly different definitions of fun, and for many people in this game, getting that new piece of gear is fun, subjecting themselves to whatever path is most efficient to achieve their goals.