I suspect most of the fraud by volume is going to be committed in the lower leagues in most nations just do to more access to individuals and less scrutiny.
The problem is any fraud at all, and the fact that it’s so incredibly easy to do it now, degrades trust and the sense of fair play. It’s now much more believable to question refs making bad calls against one team or another because of a profit incentive.
Not to mention the allegations of match fixing. As a massive college football (American) fan, every outcome that someone doesn't like is questioned on every forum as "the refs had a bet on it" or whatever.