Problem with airports like Paris, and Heathrow, is they're a pain to get from terminal to terminal and making the lounge easily accessable to only a portion of travelers diminishes the value, or the point, of having a Centurion Lounge.

If you look at where Amex has chosen to put Centurion Lounges in the US, they tend to have most or all terminals connected airside (LGA being the clear exception). I've opined on this forum before that that reason is probably a large part why LAX, ORD, and JFK don't have Centurion Lounges, seeing as how they tick the no brainer box pretty easily for where Amex would seemingly want to have a presence.