It's 100% a protocol thing. There's no firehose with AP. You only get posts from instances you've subscribed to and only from people followed by someone on your instance, and only posts since federating.

Feeds are a fundamental part of AT and depend on relays. There's no equivalent in Mastodon. It has relays, but they're more niche and restrictive and there's no standard way to filter, select, order, etc incoming posts.

The firehose used to be easy to get to on Twitter until they realized people would pay for it, and then all the fun mashups went away. The firehose and feeds that filter it are a fundamental aspect on AT.

It makes more sense once you've tried it. What we're seeing with Bluesky feeds is what Twitter could have been if they hadn't tried to turn the firehose into a profit center. It's hard to explain because it sounds like tags or saved searches if you're trying to relate it to the more common model of social media.