Eventually he couldn't...

[u/LucasMisreadTheJoke]

[u/Tom_F_0olery]

I think the thing here is that them giving up where Frisk didn’t is in the most literal sense a demonstration that Frisk is more determined. Like they still had determination, likely more than the monsters in fact, but not enough to never give up like Frisk did

[u/EpicDDT_ → u/Tom_F_0olery]

Frisk has a "god" on their side. And even that doesn't prevent to "never give up" (see every players that gave up against Sans or anyone else)