By definition, method acting is not about being becoming the character full time.
In order for actors to create natural performances, Stanislavski believed that they needed to use personal experiences in order to imagine how their characters are feeling. They would use their own memories and relate these emotions to their portrayal of a character.
Method acting means that you have to get into the mindset of the character and feel the same emotions, think in the same way and act in the same way. This is the opposite to traditional acting, which means just pretending to be someone else. In method acting, you adopt someone else's mindset.
So, if an actor wants to stay in the role off-screen and be called by the character's name, it is only to make it easier for them, to maintain the mindset of the character. Some actors might find it easy to transform from their mindset to the character's mindset, so they don't need to stay in-character off the screen. For some, the process might be quite long, so they prefer to stay in-character off-screen to feel even more in-character and don't get out of the zone.