The Green's have no infrastructure, disappear outside of presidential election cycles, make no real attempt to do things like run against Democrats in Safe seats that Republicans leave unopposed, ect. They exist only to soak up money, votes, and media attention from leftist causes that actually DO things.
They also exist to give far lefties a candidate they can feel-good about voting for. "I didn't vote for those evil corporate Democrats, I voted Green!" I understand the sentiment, but you and others are absolutely right that Greens as a party have zero interest in accomplishing anything. I've seen them called a vanity party, and that seems about right to me.