As others have mentioned, macOS both picks low ulimits but it also has a fun little poorly-documented limit for sandboxed apps which is not queryable in any way. Unfortunately how this manifests is that you try to open a file in a sandboxed app and…it just fails. Somewhere around the few thousand range or so. If you have a sandboxed app, try opening a bunch of files and see how it handles it ;)

The rationale Apple gives is something about using kernel resources, but a normal file descriptor also uses kernel resources, so I'm leaning towards implementation laziness or legacy like many of the examples here.