There are 3 good reasons for that:

• It definitely can be implemented and tested outside of the core, so it should.
• With Scala.js-defined JS classes, this feature has lost a lot of its usefulness (GitHub reports 2 occurrences of js.use in .scala files in the wild, and one of them should really be a Scala.js-defined JS class).
• It is the only macro in the core library, so getting rid of it means getting rid of all macros in the core lib.