“Master“ implies that the contents is authoritative somehow, as in “master copy” (meaning 13 in https://en.wiktionary.org/wiki/master#Noun). “Main” doesn’t have that connotation.
When one is willing to discard that connotation, then, if anything, “default” would be a more accurate name, because the fact that it is selected by default in certain situations is really the only technical difference compared to other branches.
> "Main" doesn't have that connotation.
It has had the connotation of "mainline", a synonym for "trunk", in version control since before Git existed: https://en.wikipedia.org/wiki/Branching_(version_control)
Presumably this was originally due to the connotation of the railroad mainline: https://en.wikipedia.org/wiki/Main_line_(railway)