Git is a distributed VCS (version control system). Multiple people can work on the same projects. It’ll get changes from many sources.
Rather than writing messages that say what a committer has done. It’s better to consider these messages as the instructions for what is going to be done after the commit is applied on the repo.
So write a message like this
Fix bug#1234
Instead of
Fixed bug #1234
Treat the git log not a history of your actions, but a sequence descriptions of what all the commits do.
There is a big thread on hacker news about it. There you’ll get many more reasons behind this convention.