Yes, most development is de-facto centralized on GitHub/GitLab/SourceHut/BitBucket/etc.
The Linux kernel is not, and Git was designed by the creator of the Linux kernel to serve the needs of the Linux kernel developer community. And I am certain they are not the only ones with that workflow.
Git makes fundamental design choices that are (maybe possibly but not necessarily) good for the Linux kernel. They’re objectively bad and problematic for the majority of dev work. Which makes it really fucking shitty that the industry standardized on a tool that is bad for standard workflows.
The Linux kernel is not, and Git was designed by the creator of the Linux kernel to serve the needs of the Linux kernel developer community. And I am certain they are not the only ones with that workflow.