Because you'd have to start finding ways to back up every statement you make. Customers, in general, have a healthy skepticism for the things salespeople tell them. When you make telling customers who says so besides you as a standard part of a sales presentation, you are on your way to making many more sales, and many more happy customers.
Customers care more about the benefits of a product than they do about the features.
Take a polo shirt for example. One of the features listed on the tag is cotton. This men's polo shirt is made of combed cotton. So cotton is a natural fabric. So, natural fabrics breathe better.