Because I can.
Over the weekend I petsat, a rabbit and a cockatiel. The owner had told me to let the rabbit out and go ahead and let the bird out of the cage too if I felt like it.
The rabbit was kind of an independent lady. She would come if you rattled her box of treats, but other than that, she was aloof. The cockatiel, named Rocky, was of course a prisoner in a cage. I saw no reason to let him out, what was he gonna do that was interesting? Birds don't excite me that much. What I really don't understand is people that insist on providing birdseed to the wild ones in their backyards. So you're saying you want a festival of white poo all over? Whatever floats your boat, Chachi.
On the last day they were both quite animated, pets tend to know when their owners are coming home. But Rocky was absolutely going nuts, so I figured letting him out would be a good idea, let him fly around the place and work some of that off. He very happily flew around, pooed on things a couple of times, and lighted on my shoulder then flew away again several times. Even the rabbit got into the act, jumping on the couch and crawling on my lap.
What can I tell ya? I got animal magnetism.