They are binding on the US (they are binding under international law), they just aren’t always binding within the US on their own.

A Congressional-Executive Agreement is binding under the terms of the relevant statutory authority, and typically also is (and authorized by Congress as) the basis for binding regulation by the executive.

A sole executive agreement isn’t binding under domestic law on its own, but typically is implemented by being used as a policy guideline for executive action under other authority, or as a basis for the executive seeking supporting legislation (the latter of which may transform it, after the fact, into a Congressional-executive agrement.)