Because medieval church music has the word medieval in it, which constrains the genre by history. Folk is a very generic term. Dictionaries, music publishers, singers and just about every adult on the planet have their own idea of what they call folk.

I can't see Apple and Amazon giving a fuck somehow if calling something folk sells it and those buying it are content that they asked for folk and got what they consider folk.

Folk is half a term. Traditional, contemporary, traditional rock, easy chuffing listening if you want!