The main difference between grades in Spanish engineering schools and the rest of Europe seems to be that the grade curve is heavily compressed between 4/10 and 6/10 (passing grade is 5/10). The amount of study needed to reach a passing grade would amount to an A elsewhere. As long as 25% (IIRC) of the students pass there are no consequences for the department. If less people pass, they just award a 5.0 to everyone from a certain grade up, they don't do curves. It is not unusual for people to retake a class for three or four years in a row.
There is not much guidance from professors either, no homework or extra assignments. There is just one or two midterms and a final exam, and if you're lucky there will be official course materials to study from but that is not a requirement. Most exchange students have a pretty bad time here, but on the other hand Spanish students abroad usually ace their exams with half of the work they are used to.
A more political reason is because University prestige depends partly on the knowledge of the graduading students. So if someone said they studied in X and they had no idea what they were talking about it'd give quite a bad image for the university. It's not rare to see some students crying at the exam review but it mostly doesn't work.