There are a lot of factors that go into it.
1. Don't have to think. Just stand there, receive clear.
2. Don't have to know anything about the content. Let the person/people mercing do all of the coordination when it comes to jobs and strategies.
3. Don't have to have friends. If you are starting a group to do any content, whether it be dynamis d or gaol, sometimes the group doesn't mesh. Sometimes you get a person or two or three who are just dogcrap at playing the game and ruin everything.
4. Don't need to have any relevant job geared or leveled. The guy above me referenced the fact you can heal a W3 as a sparks WHM, which is true, but even if you can, I refer you to #3.
5. Don't have the patience. This ties them all together. They can organize and learn and get a group together which might take time. Or they can just pay somebody 20m right now and be done with it.
The obvious rebuttal to all of this is "why not just play the game instead" but the reality is some people aren't interested in the parts they think are too arduous (even if those parts aren't actually arduous).