Why don't we hack core?
I could write a massive answer here, but I'm just gonna post this link: Never hack core!

The main reason I guess is that if you hack core to do something you need, and then update it...BANG!  Your changes are gone. Lost. You could then try and roll back the code from your VCS, but seeing as you can't roll back database upgrades from Drupal core - you're looking at restoring all code from VCS, and then restoring databases from your backups. All the time you're trying to roll back your code, you'll probably then notice that your last pre-update database backup failed, and you'll swear more than you've ever sworn before.

Also - most importantly - if you hack core, then Dries and Webchick both kill a kitten :-o