It will be most unfortunate if the developer dies, becomes incapacitated, or for some other reason, chooses not to continue development. Users will suffer.
Just because it's not open-source right now doesn't necessarily mean that it won't be in the future should the creator not be able to continue. It's very likely someone that thinks ahead enough to carry through large projects like this would also have plans in place to deal with life's eventualities.
For now, perhaps they want to keep their work to themselves. Not everyone enjoys working with other developers.