Friendly matches are not a worth to see games, they were played without fierce, and we see so many tries of players and tactics.
Some games are played with subs players and the one with starters are played without the real energy of a game, no one its gonna risk a injury. Thankfully its gonna end soon this part of the season called pre-season.
Wagering on pre-season games is definitely not what we expect to happen, in positions to forecast accurate predictions. In preseason games, elite clubs mostly lose to average clubs, that's totally understandable, and it doesn't even surprised me, ready to face the outcomes of these games. In pre-season games, it's consider not important and players do participate for fun. On the other hand of the coach, he utilized his players, giving gametime opportunities for every players in his squad, also a means of selecting the first XI when the season begins.
As you said, I often see big clubs losing to clubs that could easily be beaten on strength, but the fact is that in pre-season matches like this, it becomes more difficult. They end up with a lot of draws or even defeats.
I don't deny that I sometimes think about betting on pre-season matches, especially when I have more money to gamble with. Talking about the chances of winning, then it becomes lower than we bet on the same competition, maybe I don't have to say what the reason is for me saying that, because I'm sure we have the same thoughts about this.
They don't look at the final result, what they look at is how they play, and also they don't play the main players full time, but also give young players or players who rarely get a chance to play (second-tier players) the opportunity to be on the field.