People using WordPress for sometimes may know about these official themes releases named after years, namely Twenty Ten, Twenty Eleven, Twenty Twelve, Twenty Thirteen, Twenty Fourteen, Twenty Fifteen and Twenty Sixteen. To find out more just google those terms.
Most users try them for at least once, while they start using this cms. As of now more than 3.4 million websites have them installed actively. If you check popular themes, you will see top 6 themes are these official themes. In spite of so many marketplaces and premium stores with free templates in wp.org repository, these generic blog themes are most used.