Because if there were evidence of widespread voter fraud, the people who want these laws would have produced it.

Just because you haven't seen it, doesn't mean it hasn't been produced.

One example:

http://michellemalkin.com/2012/05/15/the-hits-keep-coming-project-veritas-exposes-non-citizens-dead-people-registered-to-vote-in-nc-and-so-much-more/