It would cause a lot of drama as players fail to agree on what the creation should be or destroy what others had already contributed. Many FCs already experience problems when allowing all members free access to furnishing placement. The problems would only compound in a more random and larger group of players.

It's not that I don't think it's a nice idea (games like Minecraft and Trove already allow this sort of cooperative building) but it takes only one rebel to destroy what everyone had worked to create (which has happened times unaccounted in those other games to the dismay of those who had put in hundreds of hours of effort).