Last time I suggested that the arguments in favor of very low rates of spending by philanthropic organizations are not particularly great. But law encourages and promotes low spending rates in a few ways. For example, most states require nonprofit boards to respect donor wishes in perpetuity, and to manage the organization's funds in ways that ensure the organization's perpetual life. Federal law gives donors a tax break at the time they donate, regardless of when the organization actually spends their money. Foundations have to spend 5% of their investment assets each year, but administrative and overhead costs (including, say, a fancy office for the executive) count towards the floor; other tax rules encourage living donors to park money in the firm for long periods.
These rules seem contrary to some fiscal basics. A dollar today is worth more than the promise of a dollar in the indefinite future, unless the marginal utility of that dollar will be way higher to us later (and we expect that if we had it now we'd fritter it away). Since (as Eric pointed out in his comment) we expect that if anything future folks will be even wealthier than us, granting tax deductions now for philanthropy in the 22d Century seems a bad deal. But admittedly measuring the exact extent to which this is a bad deal — deciding on what we might call the "social discount factor" is tricky.
So let me focus on two other costs of waiting that are more tangible. One is the deadweight loss of giving. Sometimes, donors want to do the best thing for the world, but they don't know exactly how to get there. Even purely redistributive charities often deliver in-kind benefits instead of cash; they must think they know better what to do with the money than their beneficiaries. And they might! But sometimes they might not, as the folks at Give Directly argue. As time passes between when the donor writes down the permissible uses of her funds and their ultimate disposition, these gaps will get wider and wider.
Similarly, time worsens the problem of the separation of the donor's "ownership" of the contributed funds and managers' control over them. You think shareholders have it tough? They can go to a board meeting. What about the decades-dead philanthropist? I guess she…what? Sends her poltergeist to rattle the chandelier at the slacking trustees?
Nonprofit law, like other forms of organizational law, is designed to mitigate these problems. But the law is far from perfect, and long waiting times between donations and expenditures put it under great strain.