The notation OP is using is relative to the key of your song. So if your key is C major, the V chord is G. However, if your key is F major, the V chord is Bb. So it's not that there's only a few chords used in popular music, it's that there's a very consonant group of chords for any key your choose.
Also, OP is leaving out lots of ways to modulate these basic chords into more complex ones (adding a seventh step, inversions, power chords, etc.).
Finally, as with a lot of pseudo-Pareto type things, often the few exceptions are what make or break a piece musically.
Any suggestions about theory learning beyond this?
Something that would help with composition perhaps? Music phrasing? Some book to read? Something for self learning? I wish that melodies I am trying to compose would be better in reflecting what I like in music, and I whish to figure out what is missing.
I can improvise with different chords but it is getting boring and once I try to do something more comlex it doesn't reflect what I like.
I think I am missing something basic and simple but since I had no other option but to learn myself mostly it is probable that I simply wasn't exposed to something essential in theory, something that all good composers know very whell, something that allows experimenting but in a productive way.
May be there is a book that is like a bible for all composers and I simply never heard about it?
I don't have any specific recommendations and am quite rusty - but years ago when I was similarly interested I just looked up a decent undergraduate music programs course requirements and got their intro harmony text followed by intro comp text; learned a lot from that.
As it was explained to me by my musician aunt (and I've heard it repeated in other places): because Music Execs. Lots of successful pop music is uses the 1-4-5-(6) chord progression, thus, when executives are picking hit songs, they go with what they know will work.
It's like bringing brownies to a potluck. It won't blow anyone's mind, but everyone will happily eat them.
That still doesn't explain why those chord progressions became popular, and they far predate the late 20th century record industry, so it really isn't a very satisfying or informative explanation, even though it's not outright wrong (it's just a restatement of familiarity bias, which generalizes outside music).
Also, OP is leaving out lots of ways to modulate these basic chords into more complex ones (adding a seventh step, inversions, power chords, etc.).
Finally, as with a lot of pseudo-Pareto type things, often the few exceptions are what make or break a piece musically.