I do not own Undertale.

Oof. I drew this in digital art, and I feel evil doing this to Sans.

Oof. I'm listening to 'Undertale Soundtrack 84: Amalgam' and the song actually fits this pic. Oof.

So basically, the reason why Sans has remained non-fused with other Amalgamates is that he had DETERMINATION injected into him like the rest of the Amalgamates, but he was smart enough to channel that DETERMINATION into not fusing with other Amalgamates.