This behavior is required because eval exists:

  function test(text) {
    const a = 1
    function inner() {
      console.log(eval(text))
    }
    inner()
  }
  test("() => { return a }")
 

prints 1 to the console

This happens because the closure's context object is shared between all closures in a given scope. So as soon as one variable from a give scope is accessed through a closure then all variables will be retained by all inner functions.

Technically the engines could be optimizing it when no eval used is detected or when in strict mode (which blocks eval), but I guess that dynamically dropping values from a closure context based on inner lexical scopes can be really tricky thing to do and probably not worth the overhead.