I bet lots of people will contribute to open source banking software if creating a bank were not extremely regulated so creating public open source bank were legal.

In that case, banking software would be very useful for the community and people will help.

The fact that something is boring has nothing to do with it. There are few things as boring as developing operating systems like Hurd or developing Wayland and dealing all day with bugs and bugs and more bugs.

Of course, people is not going to create banking software if the only ones who benefit are private companies that give nothing in return.

There must be a good reason. Something that provides the sense of deep meaning for your work. Working for free so a private company could profit out of your work does not.