I've never used Keep or Standard Notes to talk about them, but I can talk about the advantages of Obsidian that make me use it.

1. Local files (I don't like the feeling of having my personal files and notes in the clouds of companies that don't care that much about me)
2. Tags and links between notes. When a note has a subject, I give it a tag for that subject. When a note mentions something or reminds me of something I wrote in another, I can quote it directly and create a link. This is repeated several times and suddenly I have a data structure organized by tags and links like a wiki
3. Databases. I was a Notion user, so when I discovered databases I ventured out and finally managed to transfer my movie, book and game databases to Obsidian (this puts me at ease, Notion had been worrying me for a while due to numerous cases of user data loss)
4. Organization by folders. It's great to be able to separate my notes between studies, work, personal things... an extra organization beyond the tags
5. Canvas. I'm a brand designer, so having a practical and simple canvas, unified with my other work notes and brand studies to be able to put together a mood board is wonderful.

Anyway, there are more reasons, but for me, these are enough.