Because if Federer had performed worse after 2008 and never regained #1 for a second and third streak, his score in this list would be much higher (237).
It's a measure of efficiency as Number 1. It actually measures how much a player can hold his number 1 position when getting it.
1. It is a mark of greatness from a certain
point of view. It shows the player's domination power. See how some former number 1 players had many streaks one week long, and nowadays they have less streaks many weeks long. That means their advantage to the competition is higher, their domination power increased. It's either be efficient or not. If you get the number 1 position, hold it. If you can't hold it, don't get it. It's like capture the flag.
2. From another
point of view, the ATP has kept increasing the points gained in majors against points gained in minor tournaments, so the players that win majors will always dominate other players, making fluke number 1 streaks (a few weeks long) closer to less likely. This point of view also goes in my book.