Because it is constantly updated/modified.
Why are you worried that it is being constantly scanned, it is causing performance issues (I wouldn't have thought so) ?
The exclusion will work, but exclusions aren't without risk (so no one can say it is safe), you are the one that has to decide if you accept that risk. All .js files could be hacked to do more than what is intended, in 4.8 I believe I had that exclusion set, but not since I installed 5.0, I never gave it a moments thought. I haven't notices any constant monitoring, but then again I don't go looking for trouble unless there is good reason, I'm experiencing problems, etc.