Because a year consists of 12 new moons.
Since ancient times, people have used repeating natural phenomena and processes to count time. A year, for example, is the time from the flood to the flood of the river. A month is the interval from new moon to new moon, that is, about 29.5 solar days. Approximately 12 such intervals fit into a year, so 12 months were counted in a year. The lunar year is shorter than the solar year.
The youngest island
The solar calendar of the ancient Egyptians had 365 days: 12 months of 30 days and five days at the end of the year, which were not included in any month.
