That's not true around the world, and even then, you'd need to lodge a complaint and get it resolved for it to do you any good. A lot of people don't have the financial security necessary to handle that kind of event.

Beyond that, there are people who are unlikely to come back for a second dose - homeless people, those with drug problems, etc. From a public health perspective, getting those people an effective vaccine is very important because they're generally at higher risk of getting infected and transmitting it to others, in addition to generally poorer health.