I think you'll find the answer to the first question is that regulators wouldn't let them do so in North America except in a few rural areas. The answer to the second is blindingly obvious: there's already a cable system in place and every customer would have to be poached, at great expense, from the existing system. That's what a natural monopoly means.
Fiber gives them lower maintenance and lets them offer cable modems, HDTV and video on demand. Nobody builds a cable system to offer VoIP, which despite all the hoopla is still a teensy niche business. They build it to offer broadband applications.
Widespread ADSL depends on signal processing chips that have only become available in the last decade or two. When CATV was being built, the state of the art in phone line data was T1 with very expensive equipment at each end of a not very long well line.
R's,
John