Those spinning blades probably aren't dangerous to most people. But a lot of things are dangerous to Sans that aren't dangerous to most people. He tries to keep an eye on all of them as they appear - after all, there's no telling how much she is paying attention or even how much they can. And there's no point in dying just because he accidentally walks into one that she doesn't think to move.

"we're...we're really far from home. far from the underground. up on the surface." Not that he's sure if that will matter to it now. "i don't know much else. i'm sorry."