In CS it's customary to stop citing papers at some point. E.g., lots of papers are published about Turing machines without citing Turing.
Also, absolute limitations on page count is really common in CS, and the page counts tend to be pretty low. In other areas, journals might allow for more citations or citations might not count toward page count.
> Have there been any significant CS papers published in the last ~5 years that aren't on Arxiv?
Even if not, there might be insignificant CS papers not indexed by Arxiv which cite significant papers which are indexed ;) This makes the citation counts comparatively lower if most insignificant physics papers are in Arxiv.
That said, it doesn't surprise me much that worldwide there are still more people working in physics, biology or mathematics than in CS.
Also, absolute limitations on page count is really common in CS, and the page counts tend to be pretty low. In other areas, journals might allow for more citations or citations might not count toward page count.
Also, arxiv is physics-biased.