I don't want to get too into the weeds here but the UCL was less than a decade old when Messi started his career. Barca had won and been to a handful of Finals anyway of the European Cup.
Which, it was harder to qualify for the European Cup because it was only actual league champions. Also, the Bosman Ruling roughly a decade before Messi started his pro career changed football. It consolidated talent onto just a handful of clubs throughout Europe. Then the UCL format changed to let all those clubs in for basically free (except the PL where you have to earn it, to an extent, because there's more than 4 good clubs, unlike La Liga, Serie A, Bundesliga, Ligue 1 but even PL it's easier because you don't have to win the league to qualify.)
So before mid-90s you had clubs roughly made-up of home-grown (home country at least) and the talent was spread out all over Europe, was more clubs in more leagues in Europe that could compete to be the best. After, if you joined one of the handful of clubs that soaked up all the talent, you could get near free league championships every year and be one of the handful of clubs with a chance to win the Champions League. Different eras. Messi, Ronaldo, etc., their era in Europe was easier to win trophies than it was for previous players.
C'mon man.
In over 90 years ONLY 8 countries in the entire world have won the World Cup. Argentina is one of them and they won two before Messi was even born. Only 6 of those countries, in the entire world, have won the World Cup multiples times. Argentina is one of them and they had won multiple World Cups before Messi was even born.
Only 4 countries in the entire world have won 3 or more World Cups. Argentina is one of them, with 2 of those coming before Messi was born.
Only 4 countries in the entire world have been in the World Cup Final 6 or more times. Argentina is one of them, with 3 of those coming before Messi was born and a 4th just after he turned 3 years old.
Bit disingenuous to try and sell Argentina as some sisters-of-the-poor or Washington Generals equivelent or like they have the football (soccer) history of Tonga and then Messi came along and carried them to glory they'd never witnessed before. No, Messi was born in a country that wins World Cups and he helped them win another one.
Messi grew up on an Argentine NT talent era that won the U-20 World Cup 5 times in a 7 edition span. i.e. someone like Di Maria won the U-20 World Cup, too. Also won an Olympic Gold medal, too. Won Copa America, too. Won the World Cup, too. etc. (note; Argentina has won the U-20 World Cup more times than any other country in the world, for another example of their non-poor talent level.)
Argentina won World Cups before Messi. They won the Olympics before Messi (literally 4 years prior), they won the U-20 World Cup before Messi (again literally 4 years prior) and they won Copa America before Messi, 14 times. They won the CONMEBOL-UEFA Cup of Champions before Messi played for them. And they won other major/minor competitions that Messi didn't win.
Argentina is a major footballing nation. Hence, all their winning at the various levels and their success essentially since major international competitions started.