They can't do that -- they actually have elected officeholders.
Actually, the Green Party has around 230 elected officials nationwide, including the mayor of a city in California with over 100,000 people (Richmond).
That being said, I do not see the point in having the Vermont Progressives (or the Working Families Party) merge with the Greens currently. If there was PR and party lists for parliamentary elections in the US, then it would make sense though.
Sorry, let me qualify: they actually have elected officeholders in partisan positions. The Greens have plenty of people they slip in to nonpartisan elections, but aside from John Eder (and that was only for like, 2 terms), can't get anyone elected to a partisan office.