why the Cambridge Springs is rarely played at a highest level?

A lot of top players have already played it like Magnus, Mamedyarov, Hikaru. It looks a very interesting and relayable line in the QGD, but it's not played that often on the highest level (and in general as well). Why? is there any problem with it? or just is not popular?

I really have no idea, but in the spirit of people who never let that stop them answering online anyway I'll give it a go.

When the queen moves to a5, you are setting up a couple of different tactics, such as capture of the g5 bishop and the pressure on the c3 knight to be followed with the bishop. At lower levels sometimes players may fall for traps or strategic wins due to this aggressive stance of the queen. However at the higher levels they know how to effectively thwart it and make it not so good. This doesn't mean it would be stupid to play at the highest level as it undoubtedly still holds power. But like the Dragon and Dutch, black finds it tough going against players of the highest caliber, players like this aren't going to fall for tricks or crumble under complications so easily.

Alekhine surprised Capablanca with it

https://www.chessgames.com/perl/chessgame?gid=1270221

At the club level It's not played more often because White can bypass the opening by going into the Exchange Var. The CSDefense is centered around theory from the 1904-to about 1945. Most amateurs don't know the theory unless they take on the defense.

I suspect the defense being able to be bypassed is the same reason we don't see it at the NM, IM and GM level.

Conceptually, I like it because it seems straightforward: dark square attack once White's dark square bishop gets locked out, with some clear tactical patterns to watch for. The recommended White defense of backing up the f3 knight also seems really counterintuitive.