Mostly, because the data is so scarce, and often contradictory. Unlike other fields of astronomy where the problem is a fire hose stream of information that must be disentangled and sorted out, the problem in cosmology is that we are working at the very limits of our vision, and as Hubble put it, often dealing with measurements scarcely larger than their uncertainties. And every time we build a bigger telescope, we wind up with more new questions than we do with answers to the old ones.

Be gentle with the cosmologists, they may come off as flakes, but they are working right on the frontier. And they wouldn't have it any other way.