First off, there's a huge difference between one tour and another. What some folks call tours are just highly organized rides where your luggage is waiting for you at hotel/motel accommodations they arrange for you. Likewise, for meals you may be on your own or it might be a "grill" or something like that.

The more expensive tours include luxury accommodations, which are expensive to book for anyone, and given the fact that tours can only be so large before service starts to slip, the groups are often not large enough to get huge discounts. And on the expensive tours, the food and wine, etc. are substantially better than you'll have. The high-end tours I've gone on have included private dinners at small wineries, nice gourmet picnic lunches, etc. And when you consider the fact that in addition to food of this type, you need 3 people for the week to support the kind of service, you can easily see how the cost adds up.

So the question really boils down to whether people see a value or worth in paying for that kind of experience just like some will always choose a Motel 6 over a Ritz Carlton.