Every player has the same opportunity so shouldn't it then be easier for his rivals also to be more consistent?
Yes, but the point is that the best player rises to the top at every single slam.
Back when the surfaces are different, different players would excel at different slams which made it harder for one player to succeed at every slam. Nowadays if you are the dominant player, you are going to be the favourite at 3 or 4 slams in the year. Means you can accrue more slams in less time as a dominant player. You can see it at the high level tournaments in the last 5 years. The top 4 players are contenders at every tournament, and it leaves no room for anyone else. They are splitting far more titles between them.
If the surfaces had been so similar in the 80s when Lendl was the undisputed world number one then he probably would have won a few Wimbledons and a couple more AOs and his slam count would be much higher.
In the future, if someone is world number one for the year they will be expected to win 2 or 3 slams. Back in the 90s it would be more like 1, 2 if you were exceptionally dominant.