1- You can't trademark an ordinary (correctly spelled) word.
2- You want your brand to be unique, yet easily remembered, so it has to sound half-familiar, half exotic. Who will remember that "Xerox" was derived from greek xeros, dry, for being the first copying process doing away with messy liquids?
3- Corparations want to remain flexible in their expansion of business. So they don't want to be tied to a particular type of article by their brand name. Maybe this is now the unhappy fate of Xerox. Modern names of corporations avoid association with a product, either by using an acronym of their former name (BASF is an early example), or a fully "synthetic" name like e.g. Syngenta ("born" last year).