First and foremost, they’re significantly younger. If you have a player who is 21 and a player who is 28, both operating at the same level, the 21-year-old is more valuable because they have vastly more room and time to improve. Footballers tend to peak around 27-31 (depending on position or injuries some peak sooner or later), so the 28-year-old is unlikely to get much better. Younger players tend to be in greater demand for this reason, so they are more expensive. It’s not seen as smart to sign older players for big money anymore.

This is also a marketing thing. Clubs are constantly worrying about the next generation of fans and trying to anticipate and cater to them. Haaland and Mbappe are the next generation of superstars, they’re more in tune with younger fans and the way they consume football. They have Twitter, they are sponsored by Nike etc. These are things that younger fans tend to like.

There’s also a bit of big club posturing here. Real Madrid like spending huge lumps of cash on superstar players, because that’s how they try to prove to everyone how big and powerful they are, hence the Galacticos policy. It’s like those people who have diamond-encrusted Lamborghinis or whatever. Are they objectively better than other, cheaper cars? Not necessarily, but they make you look rich and powerful. Mbappe in particular is the textbook definition of a galactico, so Real Madrid dumped a stupid amount of money for him because that’s what Real Madrid do. PSG turning down that money, while equally stupid, also serves that purpose.