Because in terms of winning the very biggest races winning just LBL is heads and shoulders above winning the Giro d'Italia and the Tour de France.
Gilbert was dominant, but in terms of results and being very consistent and winning monuments and classics, it's not unheard of, and I think it's closely rivaled if not bettered by a large number of classics centered season. The only thing that stands out is the number of classics won below the monuments. In the grand scale, those are 2nd tier races at best. If I am to compare that to Sagan's 2016, I'd have to choose between a WC and CSS and 3 races the size of Quebec, and that's not taking into account Sagan winning more Tour stages, Sagan winning the green jersey, and Sagan winning the Euro's
Other seasons that are very close in terms of achievements are Boonen in 2005 and 2012 (2005 is definitely better), Cancellara in 2010, GvA this year, Kwiatkowski this year. The list is big.