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

[Akiza]

Most FFs give you 4 characters. A few give 5. Others only gave 3.

In most FFs, summoners were simply aoe nuke spells (or an aoe buff or aoe heal/res). It didn't summon the entity as a separate character you could control. FFX is the first one I can think of where the summoned being is used directly rather than "summoned for one turn only" and it was stronger than the party as a whole, in theory (depended on the exact summon used).

In an MMO a permanent pet can't be stronger than a party as a whole that would insanely op. SE can make the Summoner have a Dot phase and a bursts phase by transforming into Primals.

Summoner still should have higher single target damage then the Black Mage unless SE goes the WoW route and have the Summoner repent for turning to the Dark Arts and return to their spiritual routes like how some Warlocks repented and converted back to Shamans.