The simple reason, OP? There's no incentive to kill trash mobs. You get nothing from them.

That said, I'm not excusing people using DF right now to run the first two bosses of Brayflox HM then dropping or asking to be kicked. That's not the purpose of DF.

That's a symptom of design issues though. Putting a ridiculous myth requirement to upgrade a weapon is the main cause of this issue. Again though, people who want to speed run for myth would have no issue using the PF to do so.

The other issue is tying huge myth rewards to Roulette. It doesn't have the effect SE wants. It turns into a daily chord that you just want to get over with as soon as possible.