Sperm outlive the assumption the calendar method rests on.
Sperm can survive in the female reproductive tract for up to five days. That alone breaks the arithmetic: intercourse well before ovulation can still result in pregnancy, and it does. Pregnancy during menstruation is uncommon but entirely possible, particularly for people with shorter cycles.
The larger problem is that ovulation is not punctual. A study following 213 women found that fewer than a third ovulated between days 13 and 15, and that on every single cycle day from day 6 to day 21 at least some women were in their fertile window. Even women who described their cycles as regular showed substantial variation.
