Because most masks you can't see your mouth or nose even with minimal gaps, and masks aren't generally protecting YOU they're just preventing your spittle from your mouth from getting in people's eyes and shit. So it's bringing up an argument nobody is really making in order to dismiss mask use at all while focusing on a hyper specific example.

I wear an N95 that fits perfectly with no gaps and I still don't expect it to do anything to protect me, it's to lower the odds of getting someone ELSE sick from me.