Here is a simple four-candidate election scenario which demonstrates that a runoff is not equivalent to IRV:
% of voters Their ranking
28% W > others
27% X > Y > others
23% Y > Z > others
22% Z > Y > others
With IRV, Z is eliminated, transferring 22% to Y, who then has 45%. X is then eliminated, transferring 27% to Y. Y then wins by 72% to 28%.
With a runoff, W and X go to the next round, with 28% and 27% respectively. So the winner is W, not X as was the case with IRV.
Further, many people like having the opportunity to focus on just two candidates, so they can get to know them better, and hopefully make a more informed decision than they could have with a more crowded slate. Thus, even when IRV and runoffs feature the same two candidates in the final round, they may bear different outcomes.