Heya
First of all, I would say that writing to your representative/congressman is one of the most important things you can do. We need to fix this. If we could convince politicians to spend our tax dollars on foreign aid to a meaningful degree, that would be the same as us all agreeing to donate to charity, with the richest agreeing to donate the most.
Secondly, I would like to point out that "There probably is no God , so enjoy your life" is agnosticism at work, not atheism. It's not particularly important, but I don't like talking about things when the definition of basic words is not common. Atheism is "there is no God". Agnosticism is "there is no proof for God". There are extremely few atheists in the world.
And no, we agnostics/atheists do not particularly group together. It is perfectly acceptable to say "I started this charity for the glory of God" or "I started this charity because God/the bible" It is a bit weird to say "I started this charity for the glory of the (probable) lack of existence of God" Live aid, as an example, was run by two non religious people, but it is not thought of as an agnostic charity.
As for why such a large proportion of charities are run by churches, or Christians, as opposed to by Agnostics, or people of other religions.
http://en.wikipedia.org/wiki/Zakat - Muslims seem quite charitable. Maybe their attention is simply focussed in different countries.
Agnosticism is a very new thing compared to Christianity. We are also few in numbers, particularly in the US. This means that we have not had time to establish all that many 'atheist' charities, there's fewer of us to donate/run charities. If you compare the numbers of new charities springing up, I would suspect the majority would be non religious.
