That's just how instancing works, I'm not sure if it's that easy to change. I mean of course it could be done but the way I understand this works...instances are different from open world. There's one(well, in reality several due to it being split into shards to avoid overpopulation, but still) open world for all of us but there's N number of instances of, say, Wayrest Sewers, N being the number of teams currently doing them. After people leave, the instance disappears and as they enter again new one gets created. Of course they could keep the instance open but...
1)how would you actually reset then? On timer? What if you're farming a dungeon and want to restart as you finish? On prompt, be it by npc or in settings? Possibly, but...
2)what if you forget/neglect finishing the instance and don't go back in for another few months? Now we have an empty yet active instance that the server has to keep track of. For indefinite amount of time. And given you won't be alone with this, we will now have thousands of them...
3)how will the instance even be tracked? There's (usually) 4 people in a team and the instance is linked to a team, so as soon as team no longer exists(group disbanded), the instance kicks everyone and stops existing as well. In the case of keeping it open, would that still be one common instance(meaning server now has to keep track of everyone who was in the team) - or 4 separate ones(meaning we now have 4 extra instances instead of 1) then?
The only instance that currently kinda saves your progress is vMA but even that...first, it's solo one, second, it doesn't actually save your exact progress. It has 8 checkpoints, one after each arena, which it ticks off as you reach them. So instead of keeping full instance info in memory, it's tracking something like "char 368065689123 - arena 8". It doesn't even remember which round you were on. With a dungeon, that wouldn't fly. There're varying numbers of bosses, there're bosses people skip, there're trash packs people skip, there're different steps of a quest...it'd actually have to remember full instance.
9/10 people will wait for you to turn quest in if you notify them at the start. Personally I've also done all my first completions with friends because I wanted to have the time to read the full story as well, but that's just me. There're plenty of friendly guilds out there who do friendly dungeon runs.
How about just make the instance disappear when the LAST person leaves the instance (instead of the second-to-last person, as it is now)? That's all we ask.