Because no fake ID will EVER be 100% perfect. There will always be something wrong with it.
If you run around using it in a place where everyone is familiar with them, whatever is wrong with it is way more likely to get found. Better to take a card from a different state because errors are less likely to get picked up. Even IDchief says this, so...
BUY FAKE ID, SUPPLIES AND TEMPLATES: http://www.zoklet.net/bbs/showthread.php?p=3474509