You seem to think that "Mount" is some kind of contraction or abbreviation of the proper noun "Mountain". It is not. The full names of various famous mountains are: Mount Everest, Mount Kilimanjaro, Mount Rushmore. They are all mountains but that word does not feature in their names.
I don't know of any mountain that is actually named Mountain XXXX. There may be some, but I don't know them. They are usually Mount XXXX or XXXX Mountain or names with neither word in them.