Directory assistance is not free on most (if any) wireless plans. For example, my carrier
charges $1.99 per use.
It's been about a decade since I had a landline in my home, and the only people I know who have one nowadays are all over 60, but according the Wikipedia page you linked, calling directory assistance from a landline is not free either, and costs anywhere from $1.25 for getting a local number, to $7.95 for getting an international number.
I don't know about your web browsing habits or your financial situation, but I certanly can't afford to pay $1.99 every time I want to click on a link to a site I haven't visited before. For instance, I got home about 2 hours ago. In that time I've done laundry and cooked dinner for myself, yet I've still had time to browse web pages on 22 different domains. Had I called each of them beforehand, I'd be out more money than I pay for an entire month of Internet service, and probably used up most (if not all) of the voice minutes I pay for on my cellular plan. Though, I suppose if I'd had to make so many phone calls, I wouldn't have had time to visit so many sites.
Also, how would Google respond if I called them and asked them if it was safe to click a link to a Google Code project page? How would SourceForge respond if I called them and asked if it was safe to click a link to download the new version of the Pidgin IM client? Does Wikipedia even have a phone staff?