I only joined Zed, the company, a week ago, so take this with a grain of salt, but:

I've never really used real-time sharing (tried VS Code's Live mode and other apps) for longer than just 1-2 attempts, but at Zed things are different somehow. Everybody is constantly available in a channel and people just hop in or out. In the last 7 days I spent 3-5hrs every day pairing with others, using Zed's live mode. No video, only audio and sharing code in Zed (no video was weird at first, but now I think I'm starting to get used to it?)

IMHO it's a combination of culture and technology, but when the mix of that is right it really feels game-changing.