They might as well, though. The European Champ has won eighty percent of the Club World Cup titles and hasn't lost one in nine years, and that's a tournament with high variance since it's single-elimination.
And if Real Madrid or Man City played in a league with the MLS, Brazilian, Argentinian, etc. champs from the Americas, Asia, and Africa, they'd walk that competition every year.
It's silly to compare the NBA to European football because the PL, La Liga, and Bundesliga are far closer in competitiveness than the NBA is to EuroBasket, the ABL, etc.
No non-NBA team is beating the Nuggets in a seven-game series, which takes out the high variance of a single game elimination format. It's obvious that the best NBA team is the best basketball team in the world. Not so much with the best La Liga team.