These teams concentrate on all competitions and while it is correct that being out of competition means more resources for another competition, it is not as if they have a fixed hierarchy which competition to let go and which not. The biggest triumph is not to win one competition, but to win it all. That is the real challenge. Everyone likes to talk about having won the Champions League in year X, but being able to say you won it all, like every single trophy, is another level of respect that will come towards the players and the coaches.
If you are a smart person, you don't want to miss out on any of the potential things you can get. So why would teams do that? Because they are professionals here. Real Madrid and Barcelona have an equal chance of winning La Liga and the Champions League, and they even have a chance of winning the Copa del Rey. In terms of demand, they are fighting equally for all 3 titles and they are not willing to lose a single one. What could be more honorable for a coach than to win multiple titles in the same season? I think Barcelona can win multiple titles this season and they have a higher chance of winning multiple titles.