Many players don't feel like playing these matches at all. Of course we see that they give interviews and that they say that they are looking forward to it and that they want to win, but that is simply a moral obligation to the fatherland. Kevin de Bruyne was recently one of the few players to come out publicly saying that he had no appetite for these conference leg matches at all. Now it can be a preparation for the World Cup, but that is a coincidence.
This competition was created with the aim of ending friendly games. Therefore, it is normal for many players to take these games less seriously. As the competition is still very recent, it still doesn't generate much motivation. But with three or four more years of competition, I believe that mentality will change a bit.
There are a couple of factors at play here: the audience is also getting tired of seeing Germany vs. Italy every couple of months. Years ago that was a highlight every soccer fan was looking forward to, but these days it is not the same anymore. The consequence? Well, as a player I can imagine that it makes a difference if you know there are tens of millions sitting in front of the TV instead of just a hand full of people compared to what it is during the European Championship and World Championship. The competition itself is just not very attractive. It still has this character of being more friendly than seriously competitive. It is because people know in the back of their head that that was actually the reason why the competition was created, as you already mentioned.