Which is the more difficult feat:
1. Winning the four majors plus the World Tour Finals in a year?
2. Winning all the Masters 1000 titles in a year?
Both feats require nine weeks of high level play.
2 is still harder. 3 sets makes it easier to lose. Drop 1 set and you are on the ropes right away.
Imagine how many times the top4 dropped 2 sets in the slams over the past years. Nadal would have lost in R1 of RG if it was BO3 for example
BO5 allows a bad match and a major walkabout. Remember that Murray made all GS semis last year? Remember how many times he was 0-2 down? If it was masters he would be out, just like it happened for Murray in some HC masters and 500s.
Winning all 9 master 1000 in one year seems almost impossible. Even winning all 9 over a full career is quite crazy