Is there a reason why the Cvs or Cvs::Simple module does not have a log method?

This problem was reported 9 years ago, and there have been no updates: https://rt.cpan.org/Public/Bug/Display.html?id=15831

I'd say support for this module (Cvs) has been abandoned by the author. If you want that functionality, you'll probably need to implement it yourself.

Regarding the other module, Cvs::Simple, by a different author, you could try to contact the author for support.