Hashtags do not replace groups.
No one moderates them. They’re easy to hijack and spam. And there’s simply no permanence to them.
Which is why, if you actually want to discuss something, it’s better to tag a group. For example, if you want to be part of an actual PC gaming community on the Fediverse, it’s better to tag @pcgaming@lemmy.ca than #pcgaming.
This needs to be common knowledge because people new to the Fediverse do not know about groups. Hell, I’d say people who have had Mastodon accounts for years still don’t know. And that’s a shame.
It’s not at all clear that the developers are interested in enabling this activity. There’s no indication they care to build a fediverse omni-platform. Nor should we necessarily want such a thing.
Form follows function, and the functions of microblogs and content aggregators are significantly different on some level.