There is, in fact, a good reason as to why radiators are placed under windows. Before double-glazing was introduced and windows were mainly single-glazed, the window was the coldest area of a room. This is because the heat in the room is easily lost through the thin glass which creates a cold area close to the window surface.
By placing a radiator directly underneath the window, hot air is produced and released, which then rises to hit the cold air from the window. This creates a warm curtain of air over the window surface.
These days, most windows are double or triple-glazed, heat isn’t lost as quickly through these types of window and the area close to the window surface stays much warmer. Therefore, it’s not as common to see radiators under windows in new builds or modern homes as there can be more flexibility here.