This has only happened because the surfaces have all been slowed down. If the US Open and Wimbledon were as fast as they were in the past, there's a very good chance Rafa would have won neither of them. Look how djokovic and Nadal whinged about the blue clay as conditions there were fast. See how both tumbled out early. Why did Sampras win 7 Wimbledons and 0 French Opens? Conditions were vastly different at both. Conditions are much more similar now-a-days.
Give them credit for fucks sake. This 'only' happened because they are miles ahead of the field.
The reason Sampras didn't reach any RG finals is because he basically was all serve.
Nadal and Djokovic have everything you need to dominate, the best forehands, by far the best movement, by far the best stamina, Djokovic has the best backhand, Nadal's is certainly top 5-10.
Even a great serve (both serves are underrated), really, they have the best game for every surface, and that's why they dominate everything. They're the most complete players the game has ever seen.