During this lockdown I have been watching some sporting documentaries, including ones on Borg and McEnroe, and it seems to me that the 'obsession' with the grand slam count and it being considered as the be all and end all only really took off when Sampras closed in on the record in the mid-to-late 90s. Before then it appeared as if as wasn't such a big deal. As great as Sampras was, he wasn't a particularly big star with his grand slam finals not featuring Agassi not really generating impressive TV ratings in the US, so maybe it was a conscious effort to hype up a 'record pursuit'.
Borg voluntarily skipped the French Open in 1977 to play in world team tennis as he wanted a new challenge (the fact that it paid much better than the French Open also helped), while Chris Evert who was the best female clay court player by a million miles at time and barely lost a match on the surface also voluntarily skipped the tournament 3 years in a row from 1976-1978 to play WTT (allowing Sue Barker to capitalise one year although she has never rated her title win particularly highly). When asked about that decision, she said that 'no-one was counting back then'. It seems like Borg wasn't really that bothered about trying to overtake Emerson's 'meaningless' grand slam title record (which apparently Emerson didn't even know he held for a quite a long time) and neither was the tennis world in general, while somehow Sampras trying to overtake that record at Wimbledon in 2000 was a big deal.
And we know the Australian Open was a second rate grand slam for a while, and didn't even have a 128 player draw until 1988. For quite a few years, it appears that the year end Masters in Madison Square Gardens were considered to be important than it as were the WCT Finals in Dallas which were huge for a period, and maybe even other tournaments like Rome, Philadelphia and Wembley. And when the likes of Borg, Connors and McEnroe were playing (the golden age for tennis popularity in the US), numerous other tournaments paid more money than the grand slams. So I guess the grand slams, especially the Australian Open but also the French Open and even the other 2 as well, getting their act together and massively ramping up their prize money relative to the other tournaments, helped increase their important and therefore the importance of grand slam counting.
Slam count only really became a thing when the internet and later social media arrived on the scene. Prior to that we would just watch tennis and enjoy the beauty of it without obsessing about statistics.