Linus Torvalds answered exactly this question [0]. Not that that means you should unblinkingly take it on authority, but the original reasoning is: the renderer doesn't alway know when a line should be wrapped. Examples: a stack trace, or long log line, or essentially any other quoted artifact that has a specific pre-determined format.
The relevant quote from the link:
Some things should not be word-wrapped. They may be some kind of
quoted text - long compiler error messages, oops reports, whatever.
Things that have a certain specific format.
The tool displaying the thing can't know. The person writing the
commit message can. End result: you'd better do word-wrapping at
commit time, because that's the only time you know the difference.
EDIT: small clarification and formatting