Yes they become paragraph long. But that's the point. That's where I search for "why". And not just "why", "when, why and with what reasoning?".
Imagine if there were some interference that could derive a comment based on history and current code!
I'm certainly a fan of including lots of detail in commit messages, but often I do so and then realize "hey, this particular information would be better situated close to the code", and I move it.
Other times, yes, it's really just best to leave in the commit message because no one looking at the after-version of the code will care. For example, removal of a deprecated feature.