Simple answers:
1. Because the duty finder doesn't recognize 'support' as its own role. The last thing you want is to get four supports instead of three DPS and a support because nothing else showed up.
2. Because player expectations are already set. Most players expect both jobs to be able to *deal damage*. Furthermore, thematically speaking, there is zero replacement for someone who likes archery or guns and wants to deal damage at the same time, hence you would undoubtedly lose the support (heh) of all of those players if you dropped them into a full support role.
3. Because you would then need to design raids and possibly even dungeons to REQUIRE BRDs/MCHs or else they get dropped from parties entirely for more damage dealers. Remember that their auras scale on the number of players they are affecting, and are thus considerably less useful in 4-mans.
4. Because we only have two jobs that would actually qualify for this "support" role, which would result in a severely underpopulated role should they add 'support' to all parties and raids as dedicated slots of their own. They would need to, *at a minimum*, add another job that qualifies as 'support' to maintain the balance.