Deepseek has their own and they’re relatively small https://github.com/deepseek-ai/3FS

It’s specialized knowledge, hard to do “correctly” (read posix here) but obtainable and implementable by a small team if you pick your battles right. Also supporting very specific use cases helps a lot.

It’s also pretty easy to justify as the hardware and software from vanguard tech companies is outrageously expensive. I used to develop software for a blue colored distributed filesystem.