Well, you are asking a question about why they don't do it now: you could post this over at r/Vlaanderen and see what they say ( and yes, you don't need to write in Dutch, plenty of them know English).

But historically this was actually done once. After the Napoleonic Wars, there was treaty, worked out at the Congress of Vienna, that tried to arrange boundaries and keep future wars from breaking out in Europe. A problem in the past had been the almost constant wars between France and Germany, and it was decided that creating a country in between them would tend to keep them from fighting. Austria had authority over the Spanish Lowlands ( now Belgium) but gave it up and they were united with the United Provinces, or the Netherlands. This didn't work, for a variety of reasons. Among the more important, the Netherlands were essentially Protestant, the south Catholic. There was also a problem with representation in the government- the Netherlands ended up having much more power than the south, and was able to pretty much do what it liked there. Resentment grew until the 1830 revolt in France sparked a matching one in Brussels and Liége. Remarkably, despite some battles, this did not turn into a wholesale invasion of the south by the Netherlands. There was a peace conference in London, and Britain was the key vote for the creation of Belgium ( and would later guarantee Belgian neutrality leading up to WWI).

After 1830, Belgium would industrialize, but that industry would be mostly in Walloonia, with Flanders still being mostly agricultural. With wealth and power mostly in the south, French became the standard language of both commerce and government, and Dutch the language of potato farmers. That really did not become an important issue, though, until WWI. Soldiers in the trenches in the only unoccupied northern corner of Belgium on the Yser river had plenty of time in the first years of the war to think about the fact that their officers spoke French and they spoke Dutch. The push for equal representation in government for both Flanders and Walloonia really grew out of that time.

In the later 20th c. the famous Belgian Compromise gave equal parts of the government to both regions ( so, there's a Flemish Police, a Walloon police, and a National police) . That then became more complicated when the potato farms of Flanders were replaced by high-tech industry and the heavy industry of Wallonia became a poor rust-belt, similar to the steel towns of Pennsylvania or northern England. That turnabout in relative economic power prompted some in Flanders to consider whether they should be responsible for financing now-poor Walloonia, whether they'd be better off as their own country. That's been the main question ever since.

I could speculate that as the Flemish are pretty different from the Dutch, now, they'd welcome the idea of joining the Netherlands with about the same enthusiasm that you'd get in Toronto for proposing that Canada join the U.S.A. ( and they would really be upset at the thought of having only one football/soccer team for the World Cup: there is a MAJOR rivalry there) But that would only be a personal opinion, and also gets beyond the 20-year limit of this sub.