I wonder how far your observation is generalizable across all people. I would have predicted the opposite effect.

I tend to be much more willing to donate money to charities than to donate time. And I find this to be a general principle (ie I tend to pay my taxes without grumbling too much, but when some stupid government policy wastes my time, that's when I get angry and write to my congressperson).

Possible explanations: I grew up in a wealthy family, and/or I don't really actually spend money on anything beyond necessities because the library gives books out for free.

Informal poll: If asked to donate either one hour of free time, or your hourly wage, to a worthy charity that would receive equal benefit from either, which would you rather do? Disregard taxes being deducted from your wage and that sort of thing.