Perché così tante case hanno il finestrino in alto a sinistra sigillato?

Parce qu’il sont taxés sur les fenêtres pour sa quelle est condamnée

with the vent i assume bathroom?

You’ll see this in old houses cuz at one point people were taxed per percentage of windows were in the facade of the house. So a lot of people removed one or more windows and permanently closed them

Same in the UK Btw

Because people used to be taxed per window. According to [Wikipedia](https://nl.m.wikipedia.org/wiki/Belasting_op_deuren_en_vensters) this lasted from 1798 until 1919 in Belgium.
The upper left window is often the window in the hallway (if it’s above the front door), so closing that one had less impact than closing a bedroom window.