Thankfully, without a vessel, an angel could also only spend a limited amount of time on Earth before they're kicked back into Heaven. Hence why Lucifer didn't just blast about as a destructive wavelength of celestial intent until he found Sam Winchester and cut out Nick entirely. But, here he is, wearing an old meatsuit like an ill-fitting tuxedo.

"No, we can't. Cherubim can manifest their own forms, but they're completely solid. The rest of us must find a willing vessel-- call it divine law, laid down by our Father."