Progressive Web Apps aren't universally a good idea. I'm sharing a 6Mb/s down link with 5 other people. The absolute last thing I want is every website to pre-cache 40 pages for every page I browse.
Gatsby tries to balance making your site work at some level offline w/ not precaching too much.
By default we cache the front page & then do "run-time" caching as the user visits different pages so ensuring they can at least return to the pages they've visited when offline.