So, 5e classes generally fall into one of two broad archetypes: high at-will damage with only a small boost from expending limited resources (“nova”) or low at-will damage with a big boost from expending limited resources. The rogue is an unusual case as the only class with no ability to nova. They have no limited resources to spend, they’re all at-will damage, all the time. And with the way the game balances at-will damage vs. nova damage, that ought to mean that rogues should be the best at-will damage dealers to compensate for their lack of nova potential. But that isn’t really the case. Rogues perform about on par with fighters for at-will damage. And that’s contingent on them getting Sneak Attack every single turn.
Now, it can seem like rogues do fine not sneak attacking every turn. Part of this is because they deal very high damage per hit, and our brains focus on that, even though other characters are doing more total damage split up between multiple less damaging attacks. Part of it is because even if they miss a few sneak attacks here and there, their at-will damage output is still higher than the at-will damage output of the classes that rely on their nova abilities. But regardless of what our flawed ape brains may think, the fact of the matter is that rogues’ overall damage output is only passable if they get Sneak Attack every turn, and it’s actually quite low if they don’t.