I totally agree. Doubles is different to singles and you don't necessarily get an ideal doubles pairing by just lumping 2 top singles players together and expecting them to come up with the goods. Nadal and Djokovic and Djokovic and Murray have paired off for doubles in the past and none of them have got anywhere.
In past eras, the great doubles partnerships have been Macmillan and Hewitt (Frew M. and Bob H.), Todd Woodbridge and Mark Woodforde, 'The Woodies', Tim and Tom Gullikson, 'The Gullikson Twins' etc. None of these was a top singles player but played great doubles together and won most of the doubles titles going. The only instance I can think of where a top male singles player established a successful doubles partnership was John McEnroe with Peter Fleming and, even there, McEnroe was the only one of them who was great at singles as well. On the women's side, Billie-Jean King and Martina Navratilova won a few doubles titles together but I can't readily think of another two.
The Bryans are therefore typical of a truly great doubles partnership who dominate at what they are best at, doubles tennis while neither of them are great singles players. IMO they thoroughly deserv their reputation as the current GOATS of men's doubles tennis!