There's lots of debate about that. No-one denies that they exist, however.
Yes, but they don't come from an ethereal floating rule-book in the sky, and they (or at least some of them) aren't unchanging (or we'd still be speaking the language of Beowulf; well, really some language earlier than that).
And there are also 'proscriptive' rules of grammar which can be at odds with the language itself. E.g., some rules of Latin which proscriptivists decided should apply to English.
Which of these is right?
(1) Henry and me saw the movie.
(2) Henry and I saw the movie.
And which of these is right?
(1) Henri et moi avons regardé le film.
(2) Henri et je avons regardé le film.
'Improper' English uses the same rule as 'proper' French.
Anyway lots of non-native English speakers, including very fluent non-native English speakers, sometimes say things like 'more better'. Though, that said, this doesn't mean I won't 'correct' them (in appropriate circumstances).
[But with an increasing number of Indian English speakers (who tend to use this construction), who knows in a couple hundred maybe 'more better' will be more better than better.

]