That's an interesting point. In my years of running them all I've always needed a third-party something to upgrade or find missing media. I don't exactly know why the built-in systems don't work, but they genuinely do not seem to. I'll occasionally see a scan go off but, for some reason, nothing ever gets picked up.
So, yeah; long story short, the built-ins don't work and I don't know why and this was still easier than trying to figure it out.
Edit: if you're curious, give Fetcharr a try and let me know if it does anything for you. It's free and takes a couple minutes. It should be pretty immediate, if your experience ends up being anything like mine.
that's a decent point. Not everyone knows about the Huntarr saga (Reddit link but that's where the story broke) and what it did.
The idea is that you'll occasionally want to go through all your media and make sure it's the best quality available and that nothing's missing. New releases get published, remuxes sometimes fix issues, etc. This little CLI container goes through and periodically searches everything you connect it to, so you don't have to sacrifice hours of your weekend doing manual hunting.
Edit: as a couple have pointed out this is supposed to happen automatically with built-in searches. In my experience this isn't the case but ymmv and if what you've got works for you then that's great!