snowfalcon2
with today's computer technology it would be no big deal to design a capable integrated autopilot/autothrottle. It's about time, too, considering the technology advances in most airplane systems.
Non pilot speaking
My guess as to why these things might take longer to get more closely integrated - is the certification process. How long would it take to get approval of a new set of controls, software, interlocks and all the rest? The manufacturers will work on the basis that it ain't broke and if the customer ain't demanding it ...?