I don’t think there’s a “why.” That’s just the way we say it.
Also roommate, housemate, officemate, bedmate…but when we say “mate” by itself, it usually means a spouse or committed partner. We have lots of other words for “friend.”
We have the maritime usage, animal pairs, chess. The term just never took over here as a substitue for friend, buddy, pal, etc. The Canadians too appear immune to the habit, and the Jamaicans as well. What about the Irish and Scots? We know the Aussies & Kiwis have succumbed. I’ve seen enough Paul Hogan movies.
I think it has something to do with the Southern Baptist religious influence and the resultant negative connotations of the term “mate” when use to refer to one of the same sex as yourself.
We Americans don’t think the British use of the word ‘Mate’ as sexual, however. Its just close to mating season, mating rituals, etc for our own use. It would be a little strange and we would fear misinterpretation among other Americans.