This was a single person creation once but it grew so much bigger than that.
Also as a single-person creation, Linux is basically the implementation of an API which existed before -- not only did this bypass a huge amount of work that would go into designing a fundamentally new operating system, but it meant Linus's work would be useful because lots of existing software would run on it.
(I find it depressing that real innovation in operating systems seems impossible because everybody wants to run old software... Thus all the "bloat" has to get put back into the system.)
GNU contributed, but if they didn't exist Linux would either grow its own userspace or possibly borrow from *BSD. In the meantime, Linux did what GNU for some reason couldn't... although today HURD is actually nearly usable (in a VM) and Linux doesn't need GNU components (Alpine proves that musl/busybox can cover userspace, and with clang AFAIK you can make a completely GNU-free self-hosting system), which makes the whole thing a bit funnier to reason about.
