Why does mexico lose against weak teams?

Mexico is a very good team, they go into different tournaments winning and getting far like copa america, their first game was against brazil and they win 2-0, they have the potential to be a powerful team as well, but why do they lose in friendly matches against teams like guatemala, nigeria (tied but not good enough) and usa whose team sucks when it comes to tournaments. Do they not take the game seriously if its just a friendly? Will someone explain to me why they lose to these inferior teams but when it counts they beat strong opponents. They should play in friendlies the same way they play in a cup, they need to bring some fear into other countries.