Why is there such a long turn around time on nerfs/buffs?

I understand you want to get things right, but is there just like one guy in balance dept.? Some champs still way too strong for way too long and I just don't get how/why that happens. Look at Graves, he has been incredible strong at least this whole season and now finally there is the slightest nerf on PBE (God only knows if it will even go through though).

Jayce was really strong for about a year before he was finally turned down. Is it just an extremely labor intensive process to make changes? Or does Riot just have a limit on how many changes they want to make each week?