For one thing, good off the shelf tandems are expensive enough, let alone have a custom made one built. I rarely see a custom built tandem. 8-10k tandems are common place at a tandem rally and 90% of them aren't custom. Some go upwards of $15K and I've seen a lot of them.

Not entirely, but I've owned four and each has it's own characteristics. Pedaling a tandem is just a different feel in the legs, similar to a single but not just quite. It probably takes my wife and I a half dozen rides before our legs don't get sore after a 35 mile ride. Once we get acclimated, 60-75 mile rides are a snap.