Seafile is probably the next biggest/most developed alternative. When I’ve tried setting it up in the past I’ve had serious issues getting it running behind a reverse proxy. Would be fine for local or VPN/Tailscale style usage though. Also, data is stored in a proprietary database which would make configuring external backups a bit harder.

Syncthing is probably the most flexible option, but it’s not a “cloud” storage solution, it’s peer-to-peer file syncing. Doesn’t really allow for easy selective sync or quick deployment and configuration.

There’s also Owncloud but I don’t have experience with it.

Worse than obsidian if you try and use it like obsidian, sure. But better than obsidian at being a Notion replacement for certain tasks.

Same reason there’s dozens of closed-source note-taking and project management apps. Some have different feature sets and workflows that enable different functionality. One might be great for your specific use-case, another might not. That doesn’t mean the latter doesn’t have any merit, or that it won’t be the perfect solution for someone else who has different needs.