One thing that might be kinda "missed" here is: How much single-target damage does the Rogue deal compared to the Fighter? I think you see the difference in the Solo fight, where the Rogue suddenly deals more damage than the Fighter.
The differences can be subtle, sometimes it might not be relevant, but one thing to consider: It can be better to deal the same amount of total damage to one target than to spread the same damage around to multiple targets. Dealing damage to a single target means it is taken out sooner, and at this point, the action economy works in the party's favor - the enemies has less actions and thus less opportunities to deal their damage.
Let's say you deal 50 damage per round against 5 enemies with 50 hit points. If you could spread the damage around to 10 damage per round, it would mean that you would need 5 rounds to dispatch the enemies. If your enemies deal 10 damage each round they are active (e.g. don't start dead), that means 5 x 5 x 10 = 250 points of damage from them.
If you could deal this damage against one enemy each, you would still need 5 rounds, but each round, there would be less damage inflicted from the enemies, for example: 50+40+30+20+10 = 150 points.
Now imagine you'd deal only 25 points of damage per round, but only to one target. You need 10 rounds to deal with the enemy, giving them the opportunty to dea 50+50+40+40+30+30+20+20+10+10 = 300 damage. Despite only dealing 50 % of the damage as in our first case, you take only 20 % more damage since you target only one enemy instead of multiple ones.
Of course, I idealized the situation here - damage never goes wasted in this scenario, and we only discuss the extremes - all targets are hit, or only one target is hit. (and let's not speak of conditions!)