We do need one, but Americans, as well as many other friendly countries, have what is called "Visa on Arrival" which means that you are automatically afforded a Tourist visa just for the asking and you can get it when you arrive at the country. It's easy for Americans, who hold one of the strongest passports in the world, to forget that that visa process for many people can be a long and expensive one, even for something as seemingly mundane as tourism.

This basically adds a "pre-authorization" step once every 3 years to make sure you're not an axe murderer or fall into any other ne'er-do-well category so they don't have to watch you pitch a fit at immigration when you get denied entry.

Edit: I'll add that I pay $100 every five years (Global Entry) so that I can get back into the US on my return flight with as little friction as possible.