Understanding Judaism by Rabbi Benjamin Blech, says that another reason is that we don't say brachot before mitzvot between human and human because the delay could be disastrous (eg: starving person dies while you say bracha over tzedaka), and that by ommitting the bracha, we testify to G-d that we won't let anything delay our doing a mitzvah to save or help another person, who may not be able to withstand the delay. In contrast, doing a mitzvah between you and G-d can wait for you to finish the bracha because G-d doesn't get hungry, physically hurt, etc., so He can wait as long as you can without danger to Him(assuming of course the mitzvah isn't time-based, eg saying Shema, in which case He isn't hurt, but you still can't take too long).

I once read the following reason:

A prerequisite for the Mitzvah of charity is the existence of a pauper. I.e. in order for us to be able to fulfill this Mitzvah it requires for someone else to be in NEED. And we don't want to make a blessing, thanking G-d, for making someone else poor.