You would be shocked on both counts.
First off, the MAJORITY of bikes sold in the US are rarely used. Some are never used. My local shops sees bikes that are ridden to the point of complete failure about as often as they see basically unridden bikes coming in for a "tune-up" because 10 years in storage has rotted tires/housing.
They had a pair of Serottas come in that they sold in the late 90's for like $2,000 apiece, new in all ways with a nice coating of dust. They just weren't ridden.
On my first tour, my buddy rode a Schwinn Paramount in almost mint condition because his grandfather bought it in the 1980's after winning the lottery, and then never used it. It's a waste, and it's the norm. Not every bike sold is treasured; many are considered to be toys, or they fulfill a sudden impulsive new year's resolution that doesn't pan out, or people don't get fitted properly and stop riding because it hurts. Sad truth!
Second point, the cost. The axiom my mechanic taught me is that in order to get your money's worth on a bike build, you need to own two out of three things:
- Wheels/Tires
- Frame
- Components
If you own 2/3, it's worth it to buy the missing piece and build a new bike. If you only own 1/3, it's a better deal to buy a fully equipped bike, strip the parts you don't like, and sell them on eBay or Craigslist.
So, if I have a nice XT Groupset and I want to put it on an LHT, it's almost certainly in my favor to buy a fully equipped LHT, strip off all the stock components, sell them, and put on my own components. That will be cheaper than buying a wheelset and a frame. Dang!
If this seems like a shill, remember that the bike industry is driven in large part by service. Companies are willing to "eat" the cost of selling a bike with such huge discounts on stock parts because they hope to make back their loss through the service of the bike over a long period of time.
That's the idea, anyways. Shimano selling groupsets through third-party online vendors for less than wholesale is a gross aberration that needs to be put down.