(Contributions amounting to $1e6 over 33 years, assuming there is no history-losing VCS migration in the statistics you quoted, and a service that costs $630 per year. While 2% of development cost is still not a lot, it’s not vanishingly small, either.)
The economics are probably pretty simple: you can have fun contributing your time to a project, but you can’t have fun contributing money. It’s also easier to get your employer to okay your spending time to upstream bugfixes or even features than to get them to pay money to what’s likely a very vaguely defined organization for a very fuzzily defined service as opposed to straightforwardly buying a thing. (The money itself would be negligible—the accountants’ and lawyers’ time will cost more.)
The economics are probably pretty simple: you can have fun contributing your time to a project, but you can’t have fun contributing money. It’s also easier to get your employer to okay your spending time to upstream bugfixes or even features than to get them to pay money to what’s likely a very vaguely defined organization for a very fuzzily defined service as opposed to straightforwardly buying a thing. (The money itself would be negligible—the accountants’ and lawyers’ time will cost more.)