Potty training in the United States takes place at a much later age than it does in many other countries. In Europe many years ago, we tried to find disposable diapers for a 2-year-old. All we could find were small sizes. Later, we learned that, in many European countries, babies are most often potty trained at 7 or 8 months, when the babies often want to get away from the bulk and the mess of diapers. What a surprise!
When they are not trained then, the diapers become part of their reality and they may not see the value of potty training for themselves. If that sounds "out there," Google "infant potty training" for a wealth of information!
One of the most successful approaches I have seen is the use of the book (and recording, if wanted) "Once Upon a Potty--BOy" and "Once Upon a Potty--Girl". It is written for the child and gradually educates the child to the benefits of using the potty. The author has the goal of making using the potty the reward.
After many years of graduate study in child development, I learned that rewarding children with treats works for the moment but can develop long-term bad habits. For instance, there is some research that obesity may be attributed in part subconsciously seeking a food reward for every little accomplishment.
The potty books are very inexpensive and well thought out.