Because it's what the documentation says. For instance, the "Getting Started" example at https://docs.flatpak.org/en/latest/first-build.html says "call it org.flatpak.Hello.yml" - and that's Flatpak, which has always been a Unix-specific thing.

(That's the 'A', in YAML, literally.)

For those that don't want to wait for the time capsule, here is the earliest example of it (I've shortened it from the original for brevity; best I can gather this should also have been valid):

  <timesheet.clarkevans.com:record>
    <person:record>
      <id:int>
        293945</id:int>
      <name:record>
        <given>
          Clark</given>
        <family>
          Evans</family></name:record></person:record>
  </timesheet.clarkevans.com:record>

Oddly it kept the "Another" well into the evolution of not being a markup language anymore, but on the grand scale of things, didn't really keep it that long.

Probably for the same reason people don't type other 4-letter extensions, like `.ruby` or `.rust`.

I guess `.java` is the exception (no pun intended).