It's because nothing else requires as much time or effort. When it doesn't take you weeks to get that blue upgrade you wanted, or maybe months to even get a full blue set, then where is the sense of achievement?
Good point. I agree and I think this is a completely different problem all on its own. Players starting Pug groups and demanding higher ilvl gear than necessary for the raid being run.
I see that all the time "Normal Fresh HFC ...715 ilvl or no invite." It's a bit ridiculous. If I create a Pug group I make it a point not to ask for ridiculous ilvls. However, I understand that some raid leaders want to guarantee their successful or timely run and I don't entirely blame them.
I would argue that making friends in game and raiding with a consistent guild team are the best (only?) solutions to this particular issue.