Why is there always a saying of match-fixing in the World Cup?

In the World Cup, there are often some games in which the following offenses are committed, and the weak wins the strong.

20 18 world cup, Argentina and Iceland 1: 1 draw. Who would have thought that Iceland, who participated in the World Cup for the first time, could draw with the powerful Argentina, and many Icelandic players were still working part-time.

Germany lost 0:/kloc-0: to Mexico. It makes people feel incredible. The strength difference between the two sides is not a little. On the surface, the strong team must have won.

After watching these unpopular games, everyone has a feeling of playing fake football in the World Cup.