You’re not.
While Roon obliterates PlexAmp in UI/UX, and I’ll take RAAT over DLNA every day of the week, PlexAmp’s local discovery features are years ahead of Roon’s.
I’ve also found PlexAmp nicer to use on the go / in the gym than ARC is - just seed something and it’ll keep going in a coherent way, which I haven’t found Roon to do with my only-local library (and yes, tried Valence with Qobuz, and was deeply unimpressed).
My guess is there’s a mix of quite a bit of sunken cost fallacy, and that RoonLabs can’t get themselves to admit that Valence isn’t that great, and technical or licensing issues with doing what Plex does: I’m not sure it’s possible for Roon to do the type of analysis Plex does with streaming services.
P.S: there’s a related feature request right here. I’m not holding my breath on it being implemented.