* this is a common pattern of settlement
* the different geographies of two countries make a very simple (population / land area) density figure misleading, because Belgium is nearly 100% riverbank or floodplain and Pakistan is not
This is such a known problem that the US Census already has a different density measurement method, population-weighted density: https://www.smartcitiesdive.com/ex/sustainablecitiescollecti...
https://www.researchgate.net/profile/Kobe-Boussauw/publicati...
As a result, the region has a population density of 240/km² (only slightly less than Pakistan) but about 2/3rds of the population is in this valley representing 1/16th of the area of Wallonia. Flanders is mostly flat and more evenly settled.
This is actually more similar than I expected to the population distribution in Pakistan, with Punjab being a flat plain with an evenly distributed population, and the rest of the country being mostly settled along one river. By the way, Belgium also suffered a large flooding event last year in this valley, although nowhere near as deadly as what is happening in Pakistan.