Well... it's cause they can.
Being a BRD with 4 songs can really inflate your ego...
In the exact opposite way that being a SMN can deflate your ego...
They are a rare commodity because there's nothing else like them in the game, and have been a hard staple in endgame events since the beginning of endgame events. (though apparently SE wants to make GEO more the level of BRD? Ain't ever gonna happen. GEO's limit will always be 2 buff OR enfeebles. The only way GEO can gain more effects would be more self-only... cause the luopan abilities screw up the coding if there's more than one.)