Why doesn't congress stay on topic?

I've been thinking about this for a long time but the recent passage of the Credit CARD Act of 2009 prompted me to write it down. Would our government be better or worse if members of Congress were compelled to deal with issues one at a time? Or maybe not even that--members of Congress do need to multi-task and deal with many issues in a day. Would public policy/legislation be better if it were always limited to a single topic? We would end up with more bills certainly but would they be better?

Regardless of how one feels about credit card regulation or carrying guns in National Parks these topics are not in any way related. I know that there is a good deal of horse trading that goes on to get legislation passed. And I have heard the old saw about laws being like sausage. But even if the deal making includes unrelated topics it seems like the legislation itself should be on a single or closely related set of topics. Maybe there are big advantages to doing it this way that I just don't see but it seems ludicrous that such muddled thinking and writing are the norm and that everyone accepts it.