Open-source is one of the major concepts in the cryptocurrency space as the whole industry basically grew up on it. Nowadays, building a crypto project without publishing at least a part of your code on GitHub is considered to be bad manners.
Most of the developers involved in open-source projects are driven by the same goal: create a product that they will use themselves. This way, you get deeply involved and know about every single issue at stake. Could you think of better motivation?