How come there are so many pejorative terms in the English language which contain the word 'Dutch'?

Question

Like : Dutch treat + go Dutch, the Dutch act, Dutch uncle, Dutch courage...

Answer 1

for many years ( and this was a couple hundred years ago) both the English and Dutch navies were considered top of the list in sea-faring abilities. Witness the East India trading co. ( from England). Or the islands known as the Dutch East Indies, nowadays Indonesia.
the two nations also conceived a hearty rivalry for one another.
In slightly more modern times, as in the colonial period of the United States, the term Dutch was most often incorrectly used for Germans or "Deutsch", as in Pennsylvania Dutch. In that time where varied sects came over to what was not yet America to further their own religious beliefs, some of them could well have stepped on each other's toes, and started the name-calling on that basis.
Nothing new under the sun, eh?

Answer 2

Also things like 'French Courage'. These things go back to old common profiles, often the result of political tensions.