Why all the hate on dual wielding?

[Boci]

He said non-optimized. You're using Feats, which is pretty much the only way you can optimizing in 5e, other than multiclassing.

So GWF Fighter is doing 4d6+6 re roll 1,2 and has their bonus action but typically nothing to do with it. TWF is doing 3d6+9 but using their bonus action. Damage is 22.6 vs 19.5. At level 11 it becomes 6d6+12 re roll 1,2 vs 4d6+16, or 36.9 vs 30. Also AC is probably 17 (Splint) for 2H vs 15 for TWF (Studded Leather). Otoh Stealth is likely -2 at disadvantage for the 2H vs +3 or +5 for the TWF.

Rangers instead are looking at, if they are willing to use Hunter's Mark and already have the mark on target so the bonus is available for TWF:
TWF 6d6+1d8+9, 34.5 damage
Archery +2 to hit (or ~+20%dpr relative to TWF), 3d8+2d6+6, 29 damage (adjusted for relative dpr) at up to 150 ft.

(Note there's a lot of assumptions. Especially the bonus action for moving hunters mark competing with TWF. But also that they have colossus slayer.)

Collossus slayer only triggers once per round, so two weapon fighting is an extra chance to trigger it, but as long as at least one attack hits, its not any better for them.

Also since the fighters aren't taking feats, they are likely increasing their stat mod to +4, unless their race isn't adding to their primary stat.