After seeing all this bad takes on Belgian history reappearing again, here a very short history lesson.
Belgium's modern shape can be traced back at least as far as the southern core of the medieval Burgundian Netherlands.
These lands straddled the ancient boundary of the Scheldt that had divided medieval France and Germany, but were brought together under the House of Valois-Burgundy & unified into one autonomous territory by their heir Charles V, Holy Roman Emperor, in 1549 (Pragmatic Sanction).
The Eighty Years' War (1568–1648) later led to the split between a northern Dutch Republic and the Southern Netherlands from which Belgium and Luxembourg developed. The southern territory continued to be ruled by the Habsburg descendants, at first as the Spanish Netherlands.
This 'diamond' form is what you get when connecting the most important places of Western Europe: London, Amsterdam (Randstad), Rhine-Ruhr, Frankfurt Rhine-Main, Luxembourg and Paris.