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

[ViviAnimus]

On a single target stationary fight I'd expect BLM to do more.

True to a certain extend. Moving as a BLM isn't hard, at all. Even in movement heavy fights BLM kinda is better than SMN. BLM is imo better on every turn in Final Coil. And as I stated, the BLM sustain is a lot higher, SMN dps would still drop, no energy drain used to get MP back and BLM always has Selene. It is very obvious that SMN needs a potency buff to the extend that they should have higher ST. They simply don't.