But the difference in quality and performances between the top 6 teams in the UCL and the 20th ranked team is bigger than the difference between rank 6 in Fifa and rank 20, Qatar were the worst squad of the World Cup, but they weren't "Viktoria Plzen on a group with Bayern, Inter and Barcelona" bad.
Belgium can be ranked 1st but they can't sign 4 defenders to challenge for the World Cup, Brazil or Germany might have a stacked team but they're not drilled to play together 9 months a year.
There's exceptions like France who have been working with the same squad and tactics for almost 5 years now, but in a World Cup you don't get more than 2 or 3 teams that have months worth of training over the last 4 year cycle, the World Cup is the stage on which a single player can make the biggest difference because tactical anarchy runs rampant. South American teams don't face Mbappe and Griezmann 2 to 4 times per year giving them time to adjust, there's no room to screw a game and have a second 90 minutes at home with your fans support.
The world cup turns great players into legends and unknown players into cult heroes because it's the biggest equalizer in the sport, there's no oil money, unfair TV deals or time to steer things around in the middle of a tournament by hiring a new coach or getting extra players in January, you play with what you get, for 3 to 7 games and 90 minutes the 22 players in the pitch are the less prepared they'll ever be on the 60+ games they'll play that year.
. It's objectively worse than what we see now at international level and only tedious football hipsters would say otherwise.