I routinely open a file in my editor, hit "Ctrl-c v B" for Git Blame mode, go to the line I'm interested in, and hit "Enter". Bam, there's the full commit message. From there I can can continue to trace backwards, blaming lines and reading full commit messages.
But, you know, not everyone uses Emacs and Magit, fair. How about just using "git gui blame file"? Click on a blame line, see the full commit message. This is a tool included with Git (available in a separate package in some installations).
OK, rather use an IDE? Install GitLens in VSCode. Easily accessible blame in your editor, where you can hover or click in various places to see full commit messages.
I mean, I agree in part; there are some tools which make good commit messages hard to write or find. The tiny little commit message edit box in VScode is not ideal. Lots of people use a workflow of "commit lots of crappy commits with one liner commit messages, let GitHub/GitLab squash them on merge."
But as an expert Git user who has managed to convince some teams to have a good commit message culture, if you do get people used to writing good commit messages, they can be very easy to find and read later on, there are tons of tools that make them easy to browse.