Because 2015 is just THAT weak. Sometimes just fans can't deal with reality either. No one is playing even reasonably good tennis right now for longer than a couple weeks. Berdych is 2 in the race.
Or I could argue all guys are playing equally strong, not equally weak and Djokovic raised his level.
Maybe this is strong era when even lower guys are contenders.
So, nobody challenges Rafa on clay = strong era?
Lower guys challenging top guys = weak era?
The reality is, that people are cherry picking stats to suit their arguments. They use a few isolated examples and small sample size to asses entire era?
Using Berdych one guy as proof for entire era be weaker?
So, two guys declining make entire era weak? Murray and Djoker and Wawrinka and Cilic weren't at their peak before 2010 either, so was that a weak era too?
You can't tell if there is a weak era, because you can't compare it with anything. In order to determine a weak era you have to contrast it with strong era.
And since nobody can demonstrate a strong era. Is 4 guys dominating a strong era, or are they just no goats and are equally bad.
You do know that you can have 1 ultra goat in the toughest era ever and still dominate the most? You can have weakest era ever also when top 4 are dominating. Both is possible at the same time.
Because weak players make era look stronger and strong players make era look weaker.
You do realize that if god played tennis in an era with Fed, Rafa, Nole, Pete, Laver, they would be slamless and they would look weak.
Also if Ferrer played without top 4, he would look goat.
So, how can you determine which is which? Is Federer just like Ferrer who was lucky with a weak era? Or is Ferrer goat who got unlucky with strong era?
Or is Federer god who dominated the toughest era ever? You can dominate the toughest era ever, you are goat. You can also do poorly in weakest era ever, that means you suck even more than people say.