Did you try sp_changeowner???
There is logic behind this. Think of a packaged app - say MYAPP - which installs the schema as a certain login which is aliased to dbo (remember, no one is DBO - you are merely aliased to the dbo USER in that database). Later someone adds integration to a message bus - which commonly uses polling and triggers. That message bus installation may also be done by a login that is dbo. Now when someone connects as MYAPP login, should they be able to drop the message bus created trigger??? One could argue either yes or no.