Why it is common for PWA for PC to not cache HTML page?

I checked two exemplary PWA pinterest.com and twitter.com . Both of them have different service worker implementation for PC and mobile web. The interesting finding is that neither of the two web sites support offline mode for PC access; and both of them support offlinemode for mobile web access.

Is there any reason for that?

IMHO, App Shell makes sense only if it contains HTML page. Otherwise, every page view has to fetch from server, it doesn't improve performance at all.