The just want to write some specific software they enjoy writing together.
The question rather is: after said software has been proven valuable and widely used by companies, and when there remain some hard and/or non-enjoyable parts to be done to complete it/enhance it (specific drivers, documentation, advanced features, etc), why don't any of the users donate some money to help?
In some cases, "paying extra" without being asked to might even be compromising your fiduciary duties.
I feel the open source movement has spoilt its "user base" a little. There's very little expectation of reciprocity. Sometimes even outright hostility toward OSS modes that try work with reciprocity (check out the linked HN thread above).
This mindset might be hard to reverse at this point. As with all "communal sharing" experiments, the true crunch time comes as the economic reality sets in.
Maybe the open source communities should register as charities then? They do charitable work after all...