Because Google is magic.
On a serious note, I believe the logic behind telling someone to Google for the Verified by Visa site instead of telling them directly what the URL is, is that people often misunderstand things over the phone and have a high chance of visiting the wrong URL if told via phone. However, telling someone to Google for it is nearly just as bad.
How can you be sure that the search result they click on is the right one? This problem is confounded by the fact that Google shows different results to different people based on a variety of factors such as tastes, preferences, geographic location, so the results that a customer care person is seeing is not necessarily the same that the customer is seeing.