European Football has rules under 'Financial Fair Play' that ensures clubs don't spend more than what they earn through the football club. 

This is designed to ensure one club doesn't spend a lot of money to out-compete other teams on wages and player transfer spend and to provide an even playing ground for all teams.

The real reason these rules are in place is to ensure the competitiveness of the leagues.

If one team spends a lot of money and becomes far better than other teams, then the predictiveness of the league increases. The probability of that team winning becomes so high that it starts to deter fans to go watch the games.

When it comes to sports, predictable is boring. A tightly competed game that either team could win, or a league that any team can win, is gripping, engaging and entertaining for all fans. 

If one team starts to dominate the league, the league loses viewership over time. So, the league tries to prevent that.

If we find ourselves in a position similar to the team with a high probability of winning, then things get boring for us as well.

This is why we always look for things that challenge us - be it in the work we do, books we read, games we play, etc. 

If we are overqualified for something, we enjoy it far less, because it is then boring.