I would say the majority of people who take mileage runs only do so around the end of the year when they find themselves a few thousand miles or a few segments short of the next status level. These folks are spending a few hundred dollars, not a few thousand (and they would likely never consider spending that much just for a higher status level as the expense/time simply wouldn't be worth it to them).

The folks who are spending several thousand a year are probably a much smaller subset. I guess you could call them professional MR'ers. I can't imagine many do it completely from "scratch". Almost certainly most will already fly a number of flights for business and leisure. Many will often spend a few days if time allows at the destination to make the longer MR's a short leisure trip. I think part of it is enjoying the travel experience and not simply viewing it as a "chore". Many also attach a certain cachet to having status beyond the benefits themselves and see attaining higher status levels as a sort of challenge/hobby. The RDM's are generally just a secondary bonus benefit, but not the primary goal. Often in can be a challenge to actually use the miles as you don't earn status miles on redemptions.