I suspect that there is something else going on as well, besides just a move towards 'instant gratification'.

Lets take an example:

Manufacturer A makes a kit, and it costs them $25 in materials to assemble the kit (buy the wood, die cut it, package it, whatever). They turn around and sell the kit to a hobby distributor for $50 in quantity. That's a 100% profit margin, and a net profit of $25 per kit sold.

Now look at the same kit assembled as an ARF. Take the $25 in materials, and add another $25 in materials for building (glue, covering materials, etc). Now pay some people in Korea or China the equivalent of $2.50 per hour to assemble it and cover it, and assume it takes 30 man hours to do this work. Now you have $125 dollars invested in the kit, and can sell it to a hobby distributor for $200. That's a 60% profit margin (less than in the first case), however now they are making $75 per kit, which is three times profit margin for the kit alone!

This kind of thing happens in all kinds of industries: anytime you can leverage cheap labor to produce a 'more' finished good, and can pass that along (and then some) to the people buying your product, you can enjoy higher profits.