Is there any class G airspace that extends to 14,500 MSL?
Historically there was much more uncontrolled airspace in CONUS. There is less but it exists. 14500 is the old continental control area, sometimes called the continental control shelf.  It was designed with the highest natural obstruction in CONUS in mind.

In uncontrolled airspace IFR operations could be conducted without ATC involvement.  I used to do this frequently.

The idea was that in the Continental Control Area, IFR ops were controlled.

For the second question, some Class G exists because there is inadequate service coverage and traffic density and the blanket classification to Class E is inappropriate.