That's where the issue is. "Yet". If he were ready already, the fee would be a lot higher. The price for potential has risen massively now. Look at what Barca paid for Dembele for example. That was what, some 105+40 Mn (euros) two years ago. It's not like prices will decline. There are barely any established stars that are traded these days given the prices. The scramble therefore is to snap up the "next big thing" before he's the "real deal". When players like Vinicius and Rodrygo (neither even in Europe) are bought for obscene sums a year before they're even ready for Real's Castilla team, how do you expect a lad who's already played in UEFA competitions to be cheap? It's not "unreal".
As a former manager of mine used to say, "Price is just the highest acceptable amount that a buyer is willing to pay. It has nothing to do with cost or anything else. If any buyer is willing to pay it and the seller is willing to accept it, that's the price".