As some1 who lived in the Netherlands for a few months last year (as an international student), I have to agree that it is common knowledge that your bike will be stolen. It is mostly a question of "when?" and not "if?".
Though i have a few opinions as to why people there seem to exclusively ride junker bikes.
a) The country is flat!!! You almost never have to ride uphill. And if you do, it is for only a few yards, in order to get over a canal bridge. Thus the desire to have a more efficient (and more expensive) bike is unnecessary.
b) There is not enough bike parking!!! As crazy as it may sound, but despite the bike parking garages (for real!!! hundreds, if not thousands right outside the central Amsterdam train station), there are not enough places to lock your bike too. There aren't many options in such a situation, and thus one relegates to leaving their bike parked outside, though locked (to itself, with a quick circular lock that locks the frame to the rear wheel). However, since this is the case for the overwhelming majority of situations, there exists a certain level of security of being as vulnerable as everyone else (a thief/junkie can only ride off with so many bikes, hopefully not yours)
c) the riding surfaces are not ideal. many of the roads and bike paths there are brick and/or cobblestone, though more the former. And i would hypothetically (i never brought over my fixie, i wish i had though once i got to class there would be no place to park it!!!) find it much more comfortable to ride a big steel tank with fat slicks.
d) lastly, since bicycling is considered a major mode of transportation (i'm so jealous!), people often hitch rides on their friends ride. More specifically, a friend will sit sideways on the rear rack while their friend pedals away. This is much more feasible on the aforementioned steel tank. An extreme example i witnessed was a mother with a baby seat (and baby) on both the front handlebars and on the rear rack, with a pair of saddlebags, one with groceries, the other with a small dog!
~ellis