Um, Archen is really the only thing Gothita traps without Knock Off or Hazard support, they both trap and KO the same Fighters with no support.
Honestly, if your team desperately needs to remove Fighters it most likely needs to be reconsidered, but Gothita needs a crap tonne more support than Diglett. They actually KO about the same amount, you are underestimating Diglett's power, while you overestimate Gothita's. It fails to OHKO Timburr one of the more prevalent Fighters as well as Eviolite Foo.
You could not be more wrong.
Without Knock Off:
236+ SpA Gothita Psychic vs. 0 HP / 36 SpD Eviolite Mienfoo: 18-24 (85.7 - 114.2%) -- 81.3% chance to OHKO after Stealth Rock
(18, 18, 18, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 24)
236+ SpA Gothita Psychic vs. 76 HP / 156 SpD Eviolite Timburr: 18-24 (72 - 96%) -- 6.3% chance to OHKO after Stealth Rock
(18, 18, 18, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 24)
Basically guaranteed after a U-turn or any residual damage. Mienfoo doesn't usually even need it. 81.3% are odds I'd take any day.
236 Atk Life Orb Diglett Earthquake vs. 0 HP / 196 Def Eviolite Mienfoo: 9-13 (42.8 - 61.9%) -- 99.6% chance to 2HKO after Stealth Rock
(9, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13)
236 Atk Life Orb Diglett Earthquake vs. 76 HP / 76+ Def Eviolite Timburr: 9-13 (36 - 52%) -- 87.9% chance to 2HKO after Stealth Rock
(9, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13)
Not even a 2HKO after Drain Punch.
After Knock Off:
236+ SpA Gothita Psychic vs. 76 HP / 156 SpD Timburr: 26-32 (104 - 128%) -- guaranteed OHKO
(26, 26, 26, 26, 26, 26, 30, 30, 30, 30, 30, 30, 30, 30, 30, 32)
236+ SpA Gothita Psychic vs. 0 HP / 36 SpD Mienfoo: 26-32 (123.8 - 152.3%) -- guaranteed OHKO
(26, 26, 26, 26, 26, 26, 30, 30, 30, 30, 30, 30, 30, 30, 30, 32)
236 Atk Life Orb Diglett Earthquake vs. 0 HP / 196 Def Mienfoo: 16-19 (76.1 - 90.4%) -- guaranteed 2HKO after Stealth Rock
(16, 16, 16, 16, 16, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 19)
236 Atk Life Orb Diglett Earthquake vs. 76 HP / 76+ Def Timburr: 16-19 (64 - 76%) -- guaranteed 2HKO after Stealth Rock
(16, 16, 16, 16, 16, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 19)
Diglett has no place trapping Fighting-types.
On top of that, "desperately needing" to remove Fighters, first of all, is not that uncommon. They are excellent checks, but it suffices to run a U-turn to Gothita as a "counter" instead of a hard switch-in like Spritzee. Second of all, don't understate it by saying it's not something teams immensely benefit from. Lots of prevelant sweepers benefit from bulky Fighters being removed (like Pawniard, Carvahna, Scraggy, Omanyte, etc) and lots of bulky mons wall almost everything EXCEPT those mons (ie. Porygon, Lileep, etc).
Psychic, Thunderbolt, Energy Ball, Hidden Power, more inclusive ability (Flying-types) and the ability to take a hit make Gothita a far superior trapper than Diglett.