Maybe go look up Jonathan Pollard. If Israel is our close friend, why would they need to spy on us?
https://en.wikipedia.org/wiki/Jonathan_Pollard
https://www.military.com/history/jonathan-pollard-was-one-of...
The same reason why the US spies on the Bundeskanzler.
An actual reason I have heard is that it is as important to have independent information on your allies than it is on your enemies. Even if it is only for better coordination.
The same reason why the US spies on the Bundeskanzler.