Commit shorthashes have a major problem: As a repository accumulates a large number of commits, eventually it will contain two commit hashes that start with the same seven characters (and have the same shorthash). After this happens, tools that use shorthashes will start to break because the commit shorthash is ambiguous (it’s no longer a pointer to a single commit). Due to the birthday problem, any repository that has at least 19291 commits is likely to have a pair of ambiguous commits somewhere. So if we waited for the actions/docker repo to have tens of thousands of commits, one of the shorthashes would eventually become ambiguous and break someone’s build.

source: R