A Mitzvah to Earn a Living?

Rav Hershel Schachter in a shiur on the topic of the halachos of earning a living (here) brings an interesting source that it is actually a mitzvah to earn a living. The Rema in Siman O.C. 245:4 is speaking about the issur to set out on a journey before shabbos. The halacha is that lidvar mitzvah it is permitted. The Rema there adds that some say that a trip for business purposes is also considered a dvar mitzvah.
Now, I would have assumed that the Rema doesn't mean that a business trip is an actual mitzvah, he just means that it has the same status as a mitzvah in terms of this halacha of going on a journey before shabbos. However, the Gra on this Rema brings a Rashi in Baba Metziah that when the gemara darshens the pasuk of "v'hodata lahem es haderech yelchu bah" - zu bais chayehem, it is referring to earning a parnassah. Thus, it would seem from this Gra that we actually have a Biblical source that one should earn a parnassah.
My only question on this is: if so, why isn't earning a living in minyan hamitzvos??