IMO it's because the league is important and incredibly challenging.

PSG, Bayern, Barcelona, Madrid, and in some seasons Juventus have such an advantage over other clubs in their domestic leagues that they can rest and rotate for league games much more than English clubs can and still expect to win the league. For Barcelona and Real Madrid there's a chance they might come 2nd in the league, but that means they only need to play their absolute best players for 3-4 games per season, instead of English clubs that need to do it most weeks. And, when it's say Dortmund or Atletico Madrid in the late knockout stages, often they gave up hope of winning the league months ago and are now 100% focused on the Champions League.