one reason, cost.
Weston UK Cougar Magnum and Hype ARTF's and all the Ripmax ones are cheaper in ARTF form than kit. The little chinese kids build them for nothing, i'd love to see a bare Cougar ,or kit, but alas no more.
However, as long as there are Old Lazy Bee kits about there will always be plans you can borrow.

we've got a company comming to our club because we're the only one left that still builds a certian old kit from plans thats comming back.