After the heart-pounding rescue, we jump forward the entire journey home and join Mark sat drinking a coffee (is there anything more emblematic of being on modern Earth?). As you'd expect, the whole "man alone on an entirely different planet saved by insurmountable odds" thing has made him a celebrity, with NASA cadets slowing their training jog to meet him. The film does make a point of him starting a new life, just like the rest of the crew; what's labelled as Day 1 isn't actually his first day back on Earth, but his first day on his new "mission" - in this case lecturing. What makes him a bit different is that he's got to live forever being a Neil Armstrong-level hero/celebrity. As he opens his lecture acknowledging, he will always be known as "The Martian", but by starting with "any questions", he's making clear an intent to move beyond that. Even the man at the centre of this epic story views it as only a part of his life. The Neil Armstrong comparison couldn't be more apt. As for why Mark isn't in the epilogue? Well, we know what he's doing, so it's not exactly like he needed to appear, but his absence is in its own way showing his new life, and highlights the bigger world that was always operating around him...
