Which actually isn't all that irrational if we think of it as a decision theory problem with diminishing returns on money - making sure that at least some of your money is used well becomes more important than gambling that all of it is used well.

Of course, given the nature of what's being dome with the money, the returns diminish much, much more slowly than we're used to; diversity shouldn't be a concern until you're Onassis-ish rich.