And how much more are you willing to pay? And I'll be willing to bet they would need to do at least some revision of the optical formula since the tube would now be physically longer to accommodate the built-in TC. Or something. Anyway, personally I don't see why you would need to switch on the TC that quickly on your super-teles, but if you use them all the time maybe you have a use-case that would be useful to a large number of users.
Here is an example - I have a 1.4 on my 500 and the bird, lion, lion chasing the impala, gets to close for the combo. By the time I can get the extender off the camera, caps back on the extender, camera reattached to the lens, rebalance on the wimberley or raised to the roof of the safari vehicle, the action is over. I may only have 30 seconds to the shot. Same reason Canon puts a 1.4 in the 200-400, adds versatility