Pavers make for an economical walkway material. In addition, they are reasonably attractive and durable, and they're readily available...
Picture of walkway made of pavers.
Not only that, but walkways made from concrete pavers
provide a uniform, level surface on which to walk. This paver walkway is another example of a straight path. Straight lines make sense for walkways whose purpose is essentially utilitarian (for carrying groceries in from the car to the door, for instance, you may not want to have to walk a winding path, regardless of its potential charm).