It sounds crazy to suggest it but is it possible you have the belt on the pulley which ends up facing the drum the wrong way. The grooves of the belt must wrap around the grooves of the pulley. Another possibility is your support rollers are worn badly and allow the drum to bounce around. I have never had a belt come off that was installed properly in over 50 dryers I have worked on in the last 3 or 4 years. How old is the dryer and was the belt replaced prior to this problem ? Also is the belt the proper size for your dryer in length and width(number of grooves---some are 4 groove and some are 5 groove)

Unless the belt has stretched a lot over years of use which is not likely it should not come off the pulley. The belts used on dryers are under a lot of tension and your idler pulley/spring may be weak . If it is a coil spring type it may have stretched. If it is a type that is merely a piece of spring steel it will likely break before it is not applying tension on to the belt. Wear does occur on the support rollers but not that much to cause a belt to jump off.