"The Rambam" vs Rashi

In every shiur I've heard, Gemarah or otherwise, when referring to Maimonides, the one giving the shiur says "The Rambam". But when speaking of Rashi, I've only heard someone say "Rashi". How did this distinction come about? Why is it that people say "The Rambam says x, but Rashi says y"?

(Same is true for the Ramban, the Meiri, the Mordechai, the Rashbam, the Rosh etc. vs. Rashi, Rabbeinu Tam, Rabbeinu Yona, Tosafos)