This is like discussing why Wimbledon is more prestigious than other Slams.
It just is. And so is the classic Grand Slam compared to other records.
It doesn't matter what these tournaments happen to be played on, and it's not about winning 4 in a row when you want.
It's about winning all 4 in one season.
That's it. The holy grail of tennis has always been the Grand Slam.
I remember my grandfather and his friends talking about Laver, and they always mentioned the fact that he won the "Grand Slam". It was not about how many tournaments he won, how many finals or semifinals in a row he managed to reach, how much time he spent at number 1, etc. Tennis has gone the way of other sports and what matters now is the statistics of quantity: how many/how much/how many times. But the Grand Slam was the big thing then and is still a (herculean) possibility now (there is continuity in this), people just don't talk about it so much because no one has managed to achieve it. It's the tennis version of the 12 Labours of Hercules, and that is why Laver transcends tennis as a myth (not just as a "star" or a "celebrity", with their quantifiable fans and "likes").
But if you tell me "well, things have changed, now it's all about Slam count", then I'll have to reply "very well, but then don't talk about GOATs, because if goals and the relative importance of achievements changes from one era to another, there is no solid basis to determine a greatest player of all time".