Do breeders get mad if you buy fancy ducks from them and you don't end up showing them?
No, I wouldn't say so. If you want to keep fancy ducks without showing them, go right ahead

Breeders can choose which birds they sell to who, but once you buy the bird it's up to you what you do with it. Personally, I would try to make sure good birds I bred went to a home where they would be bred from or shown, but if that did not happen then that's none of my business.
In Europe, they go for much less caruncling on Muscovies - the smoother the better over there - for exhibition birds. Here in Australia what gets shown are huge Muscovies with quite a lot of caruncling. People here really try to breed enormous birds. Very different to the much more compact and smooth-faced European Muscovies.
Here, the term Rouens is only used to describe huge birds with a big keel. Anything without a keel is not called a Rouen, either by show people or pet people. But in the US, the term Rouen is used to describe both. So, it's all just a matter of the standards, taste and terminology that apply in your country.
Why is it that I can never find a breeder of some fancy bird who will sell them when they're young? It makes me think they want to weed out the blah ones so they can have the good ones to themselves
Yeah, that is pretty much the way it is done. If you are breeding to show, then you want to keep the best ones. They take a long time to show their potential, so you need to hold onto them for a while. I breed Swedish, and I need to wait till 6 weeks to see how the bib will turn out. To make sure their wing feathers are all coming through properly takes about 7 weeks. So I generally don't sell any birds younger than 8 weeks. With coloured (patterned) birds it can take 6 months to see how colour will end up. To see how type (body shape) will turn out takes almost a year or even two years for some breeds. So, that is why breeders hold onto birds for a while rather than selling as day-olds. I do try to make sure my birds are nice and tame though, so, even though they are sold as almost adult sized, they are people-friendly and can be hand-fed and picked up and petted. I guess the good thing about buying a grown up bird is that you know they are nice and healthy and you can see exactly what you are getting. But I do appreciate that it can be frustrating for people who want good quality birds as ducklings. I guess the options there are places like Holderreads and also people with good quality birds who will post you eggs.