Yea, they get loose over time, and with a 2 prong there's not much to hold them in especially with the weight of the power adapter.
One workaround is a short (~18") extension cord, so your heavy adapter sits on the ground (or armrest/tray table) rather than hanging out of the plug.
Another workaround is a UK plug adapter, since those prongs fit snugly, that has a good US receptacle side.