Actually, it isn't. At 75% and above, Summoner beats Black Mage in Phantom Train, Guardian and God Kefka. They are essentially tied in Clown Kefka. Only against Chadornook does Black Mage pull ahead. At 95%, Black Mage does pull slightly ahead on some of those fights (only noticeably so in o6s). None of this account for Summoner's rDPS contribution, which is why it's popular. People are the higher levels are dropping Machinist for Summoner because the latter is simply that damn good. They certainly don't need a raise, though it's helpful.
This has remained a consistent problem for selfish DPS jobs. They simply aren't doing enough damage to compensate, though the buffs have somewhat helped. That said, perception won't change due to their one dimensional playstyle. Perhaps if the damage gap was worth gambling the player knows how to play either effectively, this may change.