This is likely due to the fact that the Oscars are a celebration of the previous year's films, and the host is chosen for their comedic timing and ability to engage the audience, not necessarily their own achievements in the film industry that year.
There have been instances where the host has been nominated for an Oscar in the same year, but never won. For example, Ellen DeGeneres was nominated for Best Original Screenplay for "Finding Nemo" in 2004, the same year she hosted the Oscars.