Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Akiza]

Black Mage has always been iconic for me. This is my first Final Fantasy game (other than My Life as King). If you were to show me a picture of a Summoner from any FF game and ask me what it was from, I honestly wouldnt be able to tell you.

If you were to show me a picture of a Black Mage from any Final Fantasy and ask me what it was from, I wouldn't be able to tell you. See the fallacy in your argument it works for any Final Fantasy Class. Every job in Final Fantasy has a history outside the series which goes back to Dungeons and Dragons Black Mage is based on the Sorcerer. The Summoner is based on a Sorcerer/Wizard that choose to specialize in summoning which in Final Fantasy is a Black Mage that focuses on Summoning Magic.