A lot of players have done these dungeons for months if not years.
We have roulette's at high level which will give us a random dungeon or trial based on category (Main story, level 50, level 60, trial, expert roulette, etc)
When you've done a dungeon atleast 50+ times, you just want it over with.
I understand that it's not nice for new players, but at the same time; would you want to watch the same cutscenes 50x and the dungeon taking 5x as long in a mindless zergfest?
It's possible that in time you will adopt the same mentality.
Also don't be afraid to say you're new to something! There are a lot of Mentors out there whom are friendly and walking knowledge bases.
I myself currently have 2-3 new players under my wing and gladly help them with their dungeons, trials or answer any questions they might have.
A lot of time Mentors are also in Dungeon runs. You can recognize a mentor by their little crowns next to the name (Be aware there are 3 different types, PvE, PVP, Crafting, some have multiple statuses though)
It is possible for Mentors to hide their crowns so you might even be partied with one, without you even knowing!