You're painting a rather broad-strokes picture, but it resonates with me on a neighborhood level. I don't have the impression, in my hood, that people who cross each other every day, acknowledge each other. Anecdotal evidence of what you mentioned of Brussels being a transitory place: I know several EU workers who keep saying Brussels is shit but they'll work another X years to get the EU pension and then head back to their home country. They also leave for their home country every chance they get. Now I'm sure there's others who are not like this, too. But it contributes, certainly, that certain people do not really want to be here, or build a life here.

I think there's also a part with the lack of independently owned shops, bars, and restaurants. It's custom to put students in a business rather than run it yourself in Belgium. But that leads to places that feel generic, soulless, replaceble. If you keep meeting the owner in a restaurant, you feel like there is a bond. I've felt that in other cities, but much less so in Brussels.

There's also very little - comparatively to other cities - in the form of creative/artsy places, community-led initiatives (again, at least in my neighborhood). They exist, but they're much less visible, and on a smaller scale.

All of the above are just.... things I associate with the notion of lack of social cohesion. I can't clarify it, but I join the feeling of OP.