Because the election is not over (not tied, in your example) until the new Congress meets in joint session and counts the electoral votes.
They thought of everything.
Well, maybe not everything. Senate only comes into play in the highly unlikely event House ties on VP. Twelth Amendment shrinks the list from 5 to three. Twentieth Amendmedment puts a patch on it, "the Vice President-elect shall ACT as President until a President shall have qualified." Twentieth, by the way actually confuses things a bit, in that it says "and the Congress may by law provide for the case wherein neither a President nor a Vice-President shall have qualified declaring who shall then ACT as President, or the manner in which one who is to ACT shall be selected, and such person shall act accordingly until a President or Vice-President shall have qualified."
Congress? Which body, House or Senate or joint session? Care to watch the bodies fly as the nine wise robes untangle that language? Whoever gets elected, pray for his health between Nov. and Christmas or the Florida debacle will seem like a tea party