Haaland and Nkunku dind't have buyback clauses. They had release clauses.
There's a fundamental difference between paying £23m to someone and paying that money back +£37m when they've developed him into a world class player vs. simply paying £60m for a release clause.
There's also the fact that a buyback is exclusive to you, the seller, anyone else has to pay more.
IMO roughly 3x transfer value is the only range where a buy back works for everyone. If you want a lower buyback clause, accept a smaller transfer fee.