Variable Frame Rate (VFR) - basic questions

[nji]

I recently stumbled upon a movie that has been coded with vfr.
Quite rare I found out, so I would like to ask:

1. Why isn't vfr used more often?
I made some tests and found out that it is MUCH smaller
for movies with lots of still scenes (as you find often
at audios at youtube).
If I build a movie (H264) with one identical image (1024 x 736)
the vfr version uses 80kB (independent number of frames).
The cfr version uses 1.3kB/frame (which gives large files
if you consider fps, or the typical "audio movies" on youtube).

2. Conversion cfr to vfr - is there an easy way?
(This question is often asked the other way round,
as if you want to use the timeline in any way in
a video editor it needs to deal with vfr
which most of them don't.)
Is there an easy way to turn a cfr to a vfr?
A parameter would be a similarity limit,
so if below should be taken as "still scene"?
Or do I have to do it all manually
(Identify the dupes (ffmpeg), build timecode file from that,
remove dupes, update the timecode, hopefully audio stays sync)?