Of course some people can do it ... the maths is just that a large chunk of people cannot. I never said NO dps get in, I just said that dps spots are rare because maths.
If you take the dungeon ratio 1T1H3D up to raids, you end up with 4T4H12D
But in reality, it is 2T3H15D ... scaling for tanks - that's 30 DPS looking for 12 spots, scaling for healers that's 20 dps looking for 12 spots.
And that doesn't even account for the massive number of non raid DPS toons that add to that problem.