They do contribute a decent bit to Kubernetes per https://k8s.devstats.cncf.io/d/9/companies-table?orgId=1, but they’re still prettyyyyyyy low on the list (31 as of this writing).
Lots of it requires proprietary tools to build, source releases lag way behind their binary counterparts, sources and headers sometimes just disappear with new versions, lots of downstream patches and hacks are required to get things working, etc.
Every project that actually tries to build Darwin (the open-source components of the OS) into a usable (headless) OS has foundered, and often the death blow is Apple just ceasing to release required sources. Whenever one such project is kinda working it seems to be just barely getting by.