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

[YanderePrincess]

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.

The point is, to really keep Summoner in line with what it has traditionally been in past FF games, it would just be a reskin of Black Mage with summoned beings showing up only long enough to blast a nuke on a target, then going away. It wouldn't even be a pet class at all, just a "Nuke things" class. Honestly, in this respect, FFXI's summoners were more akin to traditional FF summoners.