Speaking of your avg, run of the mill Sony, Pioneer, etc. home theater
receiver...

Why have built-in cable tv tuners never been introduced to these? (or
have they & Im just unaware?)
Again, obviously Im not talking about some ultra-high-end setup, just
your typical receivers that someone walks out of Best Buy with.

They are pretty much designed to be the hub of the entertainment
system.. with just about all A/V sources being able to be plugged
into the receiver, and have 1 output to the tv. So why not include a
cable-tv tuner?

Obviously w/ the cable co's trying to push everyone towards digital, a
stand-alone box is needed, but that was never the case w/ the standard
basic cable.