KEL ASK 016 is a comic and an ask answer published in 2015 on tumblr by Kelly Green.
ANONYMOUS: “Sorry, Hattie. I have to ask the hitchhikers something... Hitchhikers, why do you need to catch a ride out of the mansion? It seems like the perfect resting place for a ghost.”
Phineas explains that, while the Mansion is a nice enough place, he loves travelling, like Ezra, and that they just can't stand to be permanently stuck in one place. Ezra chimes in with the fitting motto that "a resting home is no place for a restless spirit".
“Speaking for Gus, having spent practically his entire mortal life in confinement (if the Ghost Gallery is to be believed), he feels a greater need to explore now that he’s able to. Plus, Phineas and Ezra are fun to be around.”