Dependency upgrades are maintenance work that slips. Each individual upgrade is low priority. Collectively, falling behind creates compounding risk: security vulnerabilities in old packages, incompatibilities between packages that have diverged, and eventually a forced upgrade that requires significant effort because so many versions were skipped.
The work pattern for a dependency upgrade is: check what changed in the new version, update the version pin, run tests, fix any breakage caused by the upgrade, open a PR. This is repetitive and time-consuming for a human. For a small change, it takes longer to context-switch to it than to do it.