Because everything should be made available to the public and enjoyed by the public, eventually. Copyright exist so everyone can be sure their works will be profited by them and protected during their lifetime, while also giving time for their descendants making it into a profit however they want for a while. Even patents can only be protected for a short span of the world's existance, so that everyone can make use of them.
No art should be sitting on a shelf forever, unavailable to the public. Can you imagine a world where Shakespeare's plays and Mozart's symphonies are still protected? If works were to be forever protected, we would never have seen Disney's Snow White, Cinderella, Pinocchio, Alice in Wonderland, Fantasia, Bambi (which actually made Disney go trial to prove it was in the public domain and could have been used by them. They lost, but doesn't mean they didn't try.) and the list goes on and on.
An US expert in copyright law, Pamela Samuelson, once identified eight values for the public ownership of works.
- Building blocks for the creation of new knowledge, examples include data, facts, ideas, theories, and scientific principle.
- Access to cultural heritage through information resources such as ancient Greek texts and Mozart's symphonies.
- Promoting education, through the spread of information, ideas, and scientific principles.
- Enabling follow-on innovation, through for example expired patents and copyright.
- Enabling low cost access to information without the need to locate the owner or negotiate rights clearance and pay royalties, through for example expired copyrighted works or patents, and non-original data compilation.
- Promoting public health and safety, through information and scientific principles.
- Promoting the democratic process and values, through news, laws, regulation, and judicial opinion.
- Enabling competitive imitation, through for example expired patents and copyright, or publicly disclosed technologies that do not qualify for patent protection.
This is what we gain by having works in the public domain. If they're just sitting on a shelf, we just loose culture for a company who already proffited for many years off of it.
Nonetheless, I really doubt they wouldn't try and profit anyway. When it falls into public domain and everyone can release these shorts, I don't doubt Disney would release them anyway the best they can. They would still profit.