If antibiotics will clear up my tooth infection, why do I need a root canal?

I was experiencing horrible tooth pain all weekend (so bad I couldn't sleep or eat), and finally got a chance to go to the dentist yesterday. She prescribed antibiotics and Percocet for me. I'm supposed to go back in for a root canal on Monday, however the tooth is feeling MUCH better -- yesterday I needed to take 6 percocet over the course of the day, today I've only needed to take one, and that was 12 hours ago. My question is -- if my tooth infection is going to respond this well to antibiotics after only one day, then the full course should presumably clear it up entirely. If this is the case, why do I need to get a root canal done?