The thing is, unlike other positions on the pitch, there are only seemingly two goalkeepers (Neuer and Courtois) in the world who are at the level required for all of the following clubs:
City Cheslea Paris Juve München Dortmund Atléti Barça Madrid
Since Neuer and Courtois are already at two of those clubs, the other seven of us are always going to be stuck with someone a level below. There's a real dearth in quality in 'keepers worldwide.
You can name ten, and I'd be unimpressed to get rid of Hart for any of them other than those top two.