Yes, and it's widely regarded as a problem -- for someone with rare skills or knowledge, it is usually far more valuable for them to donate money to buy time from others, rather than to donate their own time. A computer programmer really should not be making and serving soup at a homeless shelter. The same amount of time spent coding could pay for several people capable of doing the same thing.

Wikipedia can directly harness those with rare knowledge, and can do so piecemeal, in five-minute intervals, rather than by taking days at a time as even extremely short employment would require. For them it doesn't make sense to pay someone to write an article on an obscure topic. It does seem to make sense for them to pay for servers and sysadmins.

(It's true that their treatment of experts really could be better. They have managed to drive several experts away because dealing with some of the editors is just not worth the time.)

Are there other areas where it actually makes sense to have volunteered time rather than donating money?