I appreciate all of Mr Dawkins' arguments (and scholarship) up to the point where he makes of a complete leap of faith and says ' because I have found the mechanism, therefore I have proved there is no inventor'. Occam's razor says that simple explanations are to be preferred to complex ones, but doesn't actually say they're guaranteed to be true!
As to why Christians voice their opinions, I choose to draw a distinction between those who simply trot out platitudes or use the text to justify their own prejudices, and those who truly believe. To understand the latter I would suggest you compare their position to that of the scientists who believe in global warming. They can see the danger, they can see the solution, many people will nod and agree it sounds like they have a good idea but don't actually want to get involved. The scientists are worried about this world, the Christians are worried about eternity.