Two things though:
* Developers, in general, are not business people. This may be different in Silicon Valley, but in a lot of the world, programmers have spent their life concentrating on one thing, learning to program. That doesn't mean they don't just "not know how" to start a business and find clients. It means they don't even know how to do the basic, first steps to registering one, knowing what to charge, etc. Let alone the complications of supporting an open source model on top of the usual business things like tax. It is just easier to throw up a paypal link and be done with it. Yes, something should be done about this, but there you go.
* When people pay for things, suddenly they expect it to .. work. As in, be stable. And be supported. And be kept up to date. All of a sudden you go from being able to work on your favorite project to supporting clients and responding to their needs. Many programmers don't want this. Admittedly, if you're floating such a project as your main income, perhaps you should be considering these kinds of things, but on the other hand one thing that keeps an FOSS project going, and gives it advantages, is not having to worry about exactly these things. It also might have a huge disincentivising effect towards contributions; why would contributor contribute if they don't get a cut? There are specific challenges in business-oriented FOSS funding models that would need to be addressed.