***** NOT A TEACHER *****
You have brought up an excellent point.
(1) Mesdames Celce-Murcia and Larsen-Freeman in their The Grammar Book
remind us that:
the basic form of the comparative is more and the
-er forms are surface lexical manifestations of
more + adjective/adverb.
If I understand correctly what those scholars wrote,
more is always theoretically "correct," but many times
idiom (the way most natives speak) requires or prefers the
-er form. (For example -- this is I speaking, not those
two scholars -- if you say "This is more good," that is not
"bad" English, but native speakers have decided that in this
case, "This is better" should be used.)
(2) Remember, too, that sometimes you have a choice.
The particular writer may prefer the sound of either more
Tom is politer than Martha.
Tom is more polite than Martha. (Quite possibly, this sentence
might seem more formal and elegant than the other sentence.)
(3) And remember that Mr. Michael Swan in his Practical English
Usage reminds us that occasionally we must use more:
He is more lazy than stupid.
(English "ears" will not accept: He is lazier than stupid.)
Thank you & have a nice day