Partly because it only works if the user visits your site with Chrome for Android (even Chromium browsers don't work).
And partly because receiving a share unrecoverably takes over whatever the user is doing. If the user opens something from your PWA (an external link, another app) and then shares back into your PWA, whatever they were looking at gets killed and your PWA navigates to the share target.
I'm building a web app built around sharing and I'm going to have to wrap it in an Android app solely to get around this.
> Editors:
> Matt Giuca (Google Inc.)
> Eric Willigers (Google Inc.)
> Status of This Document
> This specification was published by the Web Incubator Community Group. It is not a W3C Standard nor is it on the W3C Standards Track
— https://w3c.github.io/web-share-target/level-2/
Firefox doesn’t implement it either: https://bugzilla.mozilla.org/show_bug.cgi?id=1476515
Apple raised a concern about spoofing which seems to have gone unaddressed by the spec authors in over a year: https://github.com/w3c/web-share-target/issues/109
This may yet turn into a web standard given more work, but please don’t hold up a Google spec. implemented only by Google as if it were a part of the web platform that Apple are late in implementing.