It's similar to the reason that Western civilization is the most diverse and heterogeneous of civilizations: because it eventually absorbs everything else.

Art music in the Western tradition has had over a millennium to develop, during which time it has come into contact with all other known traditions and incorporated their possibilities into its own possibility-space.

That's a compelling vision/ideal, but I'm not sure that it's a good description of the reality. Most people in 'Western art music' focus their attention on a very small repertoire - much of it from central Europe, much dating to the 19th or late-18th century. Now, I'm obviously not arguing that this traditional repertoire is wholly undeserving of our attention, but surely we would expect to see more variety than that.
For good measure, even the folks who are composing new 'Western art music' with no connection to that traditional repertoire, are hardly doing a good job of being in "contact with all other known traditions and incorporat[ing] their possibilities"! By and large, they seem lost in their own variety of free-wheeling experimentation, much of which is of no lasting value whatsoever. (That is, the results of this experimentation are by and large not popular, not musically interesting, and not even valuable as achievements of pure technical prowess!) Sure, sometimes a few people from that community manage to produce something worthwhile - Arvo Pärt; John Cage, at least for his 4'33'' - the only piece of music I'd listen to all day long! - Einojuhani Rautavaara; Kaija Saariaho, etc. So, maybe we should indeed celebrate these folks as our own star composers - the Mozart's and Beethoven's of our time. But with "mass music" at the same time having undergone a veritable Cambrian explosion of diversity since the mid-20th century, without sacrificing either popular appeal or musical interest, is this really the best we can do?