The following is my opinion, based on a few years of experience and observation:
1. Because they can get that much for their fine weapons: they are considered in the top three of AR manufacturers. (Remember your ABC's: Armalite, Bushmaster, and Colt)
2. Because they want you to just buy one that is already put together, which doesn't cost MUCH more than their kits.
3. Because most of the weapons they sell are complete: there isn't much economy of scale in their kits at this time.
I own a Bushy M4, and love it. I'd have no other, but then, as quick as I say that, I am considering another kit from J&T, this time, an M4 with the chrome-lined barrel. The cost is right, and with any good lower (found as low as $90 shipped on the Internet), can save about $200 or more versus a complete Bushy.
Consider either buying the complete rifle, or if you are dead-set on a kit, look at J&T, M&A, or any of the other kit sites.
Good luck!