In this case, Spanish team's complete superiority in European competitions for one. Athletic Bilbao, Atletico Madrid and Valencia were the three teams out of 4 in the Europa League. 2 of the 4 semi finalists in the CL were Spanish team.
So obviously teams like Bilbao, Valencia and A.Madrid aren't as weak as people make them out to be.
In addition to that, both the EPL and the Serie A have been low in quality this season, and the best teams in Italy and England have been quite poor in Europe.
But my main reason for saying that, is watching all 3 leagues and making a judgement, which obviously is down to nothing but observation. But then again, there really is no objective measure when it comes to football.