Because it's not the committer's job to play housekeeper for the contributor. They have enough work to do already, and the contributor is the one who wants their code merged in the first place - they should put in the basic effort to clean their own code before submission, just like everybody else. And because that would introduce some grey areas into the Signed-off-by line[0]. Committers do not want to change submitted code. See the bit about process for maintainers modifying submitted patches and imagine if they had to do that all the time because people can't just follow the damn style guide.

[0] https://www.kernel.org/doc/html/v4.17/process/submitting-pat...