Well, speaking in the case of the US, this would constitute product development which is well outside the scope of what a 501(c)(3) organization should be doing, which could thereby jeopardize their tax status? Or, in the case of a state-run university, this raises all kinds of issues regarding how tax money is being given away to random schmoes instead of benefitting the public at large.
So, yeah, there's plenty of reasons why they don't do that.
Open source wouldn't have a funding problem if people would stop being so averse to just paying for what they use. Maybe... the world should stop expecting something for nothing.