That's an opinion, not a fact, and one that depends on who you ask and where. If you go to the Flanders region people will certainly tell you they speak Flemish.

Language researchers can argue all they want about how languages should be grouped, but they sound different, they use different words, and people call them different things. It is simply incorrect to state that people in Belgium speak Dutch. Might as well just expand the group to include German and English (etc) and say they speak Germanic.

Fun fact, the Dutch call their language Nederlands and refer to German as Duits ("Dutch").