It's not necessary that every club has one. As long as one club has a super-rich foreign investor, then that's one person injecting money into the club and into the game. Every time an investor injects money into his club, he is injecting it into the market, because that money ends up being circulated between clubs through transfers and all sorts of dealings. Therefore, an investor isn't just increasing the value of the club he invests in; he's also increasing the value of football as a whole. Each new investor multiplies this effect.
As an example, take a look at Dinamo Zagreb. They don't have a rich foreign owner, and are relatively poor compared to all Premier League teams and probably most Championship teams. However, they have been able to sell players like Eduardo, Corluka and Modric to rich teams in this country and have made what to them is an absolute fortune. Think about how many teams like this must have increased their value significantly from selling players to teams with rich owners. Think how many teams other than Man City must have benefited from Sheikh Mansour's injections. Now they're all probably paying their players higher wages and charging their fans more for tickets. That's how it works. If a foreign investor comes in and invests hundreds of millions of pounds in a team, he's not just investing in them; he's investing in the economy of football as a whole.