The modern three-point seat belt was invented in 1959 by Volvo engineer named Nils Bohlin. His invention was a significant improvement over the prior two-point waist restraint and it's estimated over a million people owe their lives to the seat belt.
The reason for such popular adoption is that when Nils invented the seatbelt, Volvo opened up the patent so that any manufacturer could use it in their design. They decided that such a significant invention was more valuable as a free life-saving tool rather than an extra bullet point in their sale sheet.
Today, seat belts are not just a requirement in all cars, a large number of countries have adopted seat belt laws that make it a requirement to use them.