Not all on the "evolution side" do this, of course -- I've found on many threads that it's quite possible to have a great discussion on the scientific merits/problems of a design hypothesis, and also the underlying assumptions that permit or rule out such a hypothesis.
However, there are also those (one might call them "evolutionists" to denote their ideological stance) whose views are informed at least in part by an antipathy toward religion. This ideological approach causes them to build the strawman you've mentioned. It's much easier to fight a strawman.
Ironically, the rabid "evolutionists" and "creationists" are very similar in their tactics and approach. Both sides seem to view the other as a direct threat to their faiths, and seem completely unable to acknowledge the merits of the opposing position.
Other than forcing biologists and science writers to sharpen up their arguments, what has ID contributed in the way of actual science?