Also browsers can use extensions to modify pages, and that opens up SO many possibilities. And bookmarks. And history. And tabs. And search. Most apps never implement any of these, much less all.
I do wish we had a more widely supported system for push notifications though. WebPush exists and broadly works quite well, but provides nowhere near the control, reliability, and observability that native apps do. ...which is a good thing when you don't want them, but crippling when you do, and it contributes to the general "few sites do it at all" problem. E.g. Slack doesn't use it, nor does Zulip (particularly painful because their mobile web UI is much better than their app), nor most chat apps in general even though they're an ideal scenario.