Because in Catholic usage, a Saint is primarily a person who has the Beatific Vision (is in Heaven). We donāt qualify.
We recognize that some of the Biblical writers used Saint in reference to the Church on Earth, and thereās nothing wrong with that given how the word was understood at the time. Thatās just now how weāve come to use the word as Catholic language has advanced. The title of saint has developed in meaning to the point where itās no longer appropriate to apply it to those who are still running the race.
Also, by calling ourselves sinners, we are reminded of our constant need for repentance and Godās mercy.