I noticed that a lot of time people just run through the instance and stand where the boss spawns waiting for minutes.

There are only two reasons why the boss doesn't spawn:

1) Someone already killed the boss.
2) You need to kill more mobs.

Just standing there isn't going to do anything! Most bosses in this game require a certain number of mobs to be killed (don't think it matters where they are), as long as you kill a certain number.

Anyway, happy farming.