Writing A Sefer Torah

וְעַתָּ֗ה כִּתְב֤וּ לָכֶם֙ אֶת־הַשִּׁירָ֣ה הַזֹּ֔את.  The Rambam beggining of Ch. 7 from the Laws of Sefer Torah says מצות עשה על כל איש ואיש מישראל לכתוב ספר תורה לעצמו שנאמר ועתה כתבו לכם את השירה וכו.  The rosh says that nowadays that our main learning is not from a sefer torah, rather from other seforim, one fulfills the mitzvah by owning other seforim.  Mitzvot don't change, so obviously the Rosh holds the geder of the mitzvah is to has a sefer to be able to learn with.  Assuming that is the case, why could one fulfill the mitzvah of writing a sefer torah only if he personally writes it, and if one must actually write it themselves, why isn't everyone looking to print their own Gemorah?  See here.