The thing is that the Millers and Wagners of the world want to go somewhere that they can compete for a title. Sure money is important, but they both have a couple of bucks in their pocket. Miller found someone who is a SB favorite that would (IMO) totally overpay him and I wish him the best. Wagner never got that kind of offer, but in his own words
Bobby Wagner On His Next Team, 'I Just Want To Win'
The Rams give him that opportunity. The problem the Giants have is they are a not-so-good team that doesn't have a lot of salary cap flexibility. While I don't follow them closely, I suspect they've made some rather poor signing choices. It's one thing to be in salary cap hell if you've had a competitive team and have to rebuild (think Saints) and it's something else to be there and to have fielded a team that was never going to compete for a title. A regime change was in order, but you have to give them some time to build. They have some pieces and with a good draft or two, they could field a very competitive team.
Not having a winning team AND salary cap issues isn't going to get you Pro Bowl caliber guys like Miller and Wagner who have made a lot of money, are closer to the end of their careers than the beginning, and want another title.