Because giving to your church is a less effective use of charitable monies than "giving" to the government?
Okay, Dishevel - what fraction of the money contributed to a church actually gets passed on in a form that resembles what is commonly thought of as "charity"?
Many churches (most, I suspect) make the record of their finances public - so it is easy to find out what is typical for the "overhead" of this charitable church operation. Remember - your contention is, I believe, that all of the money given to a church is charity, not just some of it. This is certainly how the sources you point to analyze the data.
As a for-instance, type "church financial statement" into Google and go down the list that is returned in order. You can use some other selection method if you like, but this one can be repeated and has no in-built bias.
The first church that is returned is the First United Methodist Church of Austin. Its 2011 audited financial show contributions of $2.357 million. How much of that is passed on as charity? Well, the only part of their budget that contains items for aiding the community is called "Missions", which expended a total of $735,692. Essentially all of the rest is spent on the same church members who are contributing, mostly in funding its religious worship.
As a charity this is just an awful return - only 31% is being spent on actual charitable services. The remaining 69% is being spent on a religious social club - fine for the members, but in no way a "charity".
But wait! That's a "liberal church"! They must be tight-fisted charity givers, right? The third on the list is the Faith Promise Church of Knoxville, Tennessee, a very conservative Evangelical ministry. It has 2012 revenues of $7.9 million. Its charitable works spending? Its "programs and missions" budget is just $582,000, or 7.4%
You will find this for any church budget you care to analyze, the 31% of the Austin church really surprised me, it is one of the highest I have ever seen. Typically less than a quarter of the money that goes to a church (often much less) gets spent in any charitable fashion.
And when you make this adjustment, the "generous conservative" disappears.