This is a two part question. 1: Why aren’t the Dutch considered Nordic, they’re extremely close to Scandinavia in terms of culture, and at one point were a part of Denmark

1. The Dutch nor the (various states that later constituted) the Low Countries were never a part of Denmark.
2. The principal determining factor in whether a country is considered 'Nordic' is whether or not the population speaks a North Germanic language and is located in Scandinavia. Dutch is a West-Germanic language, unlike Swedish, Danish and Norwegian which are all North Germanic languages. Of course the Netherlands, are not a part of Scandinavia nor do they neighbor it.

Nevertheless, aside from language and geography, it is true that there are marked cultural similarities between the Dutch and Scandinavians when it comes to values. Research by the Belgian sociologist and statistician Geert Hofstede has shown many similarities (especially between the Dutch and Norwegians, less so with Danes and Swedes) when it comes to social values and attitudes.

However, these similarities have more to do with a similar political development following the Second World War rather than similar ethnic, linguistic or cultural backgrounds. In fact, in terms of language, traditional religion and many aspects of general culture; there are many differences.

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2: Why haven’t the Dutch been recognised as having Viking ancestry. It’s well documented that the Frisians played a relatively large role in the Viking sphere, they were known for their boat building ability, and even accompanied the Danes in raids on England - is that not the definition of Viking?

The Frisians (which at the time, did not exist as a well defined ethnicity) played a relatively minor role during the Viking Age; being mainly descended from the same migratory wave that left the Jutland peninsula and German bight towards England during the 5th and 6th century, but instead settled in what is now the Dutch province of Friesland and adjacent coastal areas.

The Franks, the most direct ethnolinguistic ancestors of the Dutch, were not particularly noted for their command of the sea during this or the later Viking period either; being mostly associated with river and land trading.

Though there is some evidence of Frankish raiders being active in the North Sea during the latter phases of the Anglo-Saxon settlement of England and of warriors from the Northern Netherlands occasionally joining Viking raiders doesn't so much prove them as being Vikings as well, but rather evidences that the raiders now covered by that term sometimes also included people who were not Norse-speaking or from Scandinavia; which was nevertheless true for the overal majority.