Not everyone in the progressive caucus is a socialist, otherwise...they would be members of SPUSA (Socialist Party USA).
It's true, the DSA is not it's own seperate party. During a socialist convention during the 1970's the DSA seperated themselves from the SPUSA
because they felt dominated by the two parties and understood that a third party was hopeless in America, thus they 'joined' the democratic party by
pushing more candidates that are in-line with their ideals.
The only true socialist party in America even have
their own presidential candidate. You
never hear about him, or any of the other
290+ candidates running for
president though.
DSA is not it's own political party because they would then have to raise funds and report to the Federal Election Commission. SPUSA is a political
party and have to raise funds and report to the FEC.
The Democratic party may have some members that are socialists (myself included) but that doesn't mean the entire party is socialist, I identify more
with socialists than democrats. Just as a handful of constitutionalists in the Republican party don't make the entire party the Constitution Party,
particularly because there's already a
Constitution Party.
When we only have two parties to choose from you're going to get a little bit of everything under the banners.