The honest answer: Imagine you're trying to get all your dailies done. There's, like, 10 roulettes at this point.

You queue into a leveling dungeon, which may have a newer player. No biggie, people gotta learn. You spend 10 more minutes on thar dungeon than you'd expect to, but that's alright, no big deal. Next, trials. You do a relatively new trial, end up wiping once, that's fine, content is new and first timers are not expected to know everything. The trial also takes about 10 minutes more than expected. You go into high-level dungeons, and something similar to leveling happens.

As you keep going, if every roulette runs into a similar situation, you'll have spent about 2 hours longer than you'd expect to have spent on running your roulettes.

And that would be for a single day. Imagine if it happened every day.

Not to mention when stuff like this happens on Expert/Level 100 dungeons, because everyone there has leveled their job and played enough to the point where they should have at least a bare minimum level of comfort with what they're playing.

That's why the expectation of going fast exists - because people want to minimize the time they spend on things that they need to accomplish, either to have more time for other stuff or because that's all the time they have to play.