The “both sides make their claims, who are we to judge?” reflex is very powerful in politics, even if one viewpoint is objectively true and the rest are not. Expanding upon your observation, consider how
10% of Democrats are birthers. The percentage of Republicans, however? 53%. Yes, nutty Democrats do exist, but as far as the Democratic base
and Democratic establishment are concerned, they're practically nonexistent; Democratic birthers and the like have steadily abandoned their party and vote Republican nowadays (e.g. West Virginia). This is the very different from the Republican field where the nutjobs compose both the base
and the party.