That's a figure of speech, it doesn't mean every single instance of a player transfer ever was done purely for money. I apologise if English is not your native language and that idiom was unfamiliar to you.
Edit: I see I have been ninja'd by NS.
Bit patronising.
Even with the idiom I still disagree as it still implies that this is the motivation in many/most cases, even if not literally always.
Top footballers get paid crazy amounts of money but I don't think getting even more money is the motivation for moving clubs in most cases. Players move to enhance their careers - that may be to play more, to have a better chance of winning trophies, to be at a club that will give them more opportunity to advance as a player. Being paid loads is, of course, a nice additional consequence but not the primary motivation in my opinion.
Don't forget that to even get to be a professional (let alone a top professional) requires years of massive commitment as well as talent. To imply that as soon as a player attains that level that money suddenly becomes the key doesn't really seem plausible.
Sure there are examples where player move primarily for the money, where the club they move to won't give them the other things I've described. But that largely seems to occur at the end of player's careers. So, yup, Jordan Henderson (age 33) has just moved to Saudi for the money (he's over tripled his wages), but I doubt that money, rather than the opportunity to enhance his career, win trophies etc would have been the prime motivation when he moved from Sunderland to Liverpool at the age of 21.
And if Mbappe moves to Real, or Barcelona, or Inter Milan, or Man U, or Liverpool, or even Man City etc, etc he will almost certainly have to take a pay cut as none of those clubs (even Man City) come close to being able to match PSG on player salaries.
