What exactly is AVCHD?
I think this image from Wikipedia says it best:



AVCHD is not a codec, but it is also not just a container.

Just like "every other" container, AVCHD features:


Video streams
Audio streams
Subtitles


But in addition, for example:


Playlists
Menu presentation (like in a DVD)



  Is it simply a specification that specifies a particular combination of pre-existing technologies?


Yes.


  But this would imply that AVCHD files could also be called something else, like "MPEG4 container with H264/AC3 inside".


But who would like to call it that? Several other specifications take already existing specifications, remix them, and they aren't called "Open Source Container format for XYZ".

I think it's a "one format fits it all" mentality, which is why you'll find AVCHD on many consumer devices.