Salaried professionals often cannot do an extra hour of work in order to donate the proceeds to charity. My employer basically prohibits me from moonlighting/consulting/etc. Even many hourly employees can't get extra hours at work as that would be higher-rate overtime that their employer is unwilling to pay. Monetary charitable giving takes away from my current bottom line, but charitable working just eats into my leisure hours.

Since I cannot do extra paid work without fear of consequences at my primary job, my non-work time may be practically worthless. I can only use it to do things that I might otherwise pay someone else to do. If I can do work around the house, then I can save the cost of paying the plumber. Suppose I make $100/hr (nominally) and the plumber charges $50/hr. Assuming we can do the same job in the same time, I haven't lost $50/hr by doing it myself instead of paying the plumber, I've simply lost the utility of those hours which I may not rate highly if I'd have otherwise laid on the couch watching Simpsons reruns.

Some units of caring cost more than others. I can donate $100 to charity, or I can do 100 hours of work for that charity using hours that only cost me $1/hr (presuming that I rate the utility of those hours otherwise spent low enough).

Clearly, people shouldn't be derided for donating excess money (the "overemployed"?) to charity rather than their time, but I think the calculus is a little more complex than what you describe in your post. For those living near their means (neither under- or overemployed), there are additional economic factors that make donation of time heavily favored over donation of money. That a culture of valuing this has arisen to justify/rationalize such behavior shouldn't be terribly surprising.