The amount of resources commanded by the British student unions (including student union buildings, junior common rooms, etc) is staggering compared to what student unions in Spain have available. The same thing is true in the US.

Plus, in Spain there is a difference between the sindicato de estudiantes (student union, in the "labour movement" sense of "union") and whatever it is that the "student government" is called at each school. The sindicato de estudiantes is a national organization with its own agenda, usually a launchpad for political careers and has fuck-all to do with students.

When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes

appear a very Anglo-based concept to me. Even the Dutch system, which does come with orientation weeks and a wide umbrella support system, does not work with a centralized union. Societies are school based and funded by the school and private sponsors. Politically, student representatives from different political flavours, chosen during annual elections, sit on the school and university councils/boards. Hence, it's a two-tract system.

Generally the student societies per school pull the cart during orientation week - also with the ulterior motive to grow membership.