This is an important point to consider.
For large corporations, it is easier to hire a developer to work on a critical piece of open source software than it is to go through the approval process of sending money (no strings attached) to a particular individual.
Which is quite something considering the dog and pony show that is the recruiting process of most large orgs. One suspects that if OSS developers routinely offered to invoice for "software support" rather than requesting a donation, this would be a vastly superior option from the company's viewpoint compared to having to go through the expensive palaver of hiring.
I think citation is needed here. Many big open source projects are created and maintained by the paid employees of big corporations.