Because human development can be paused before birth, and because prematurely born babies are developmentally the same age at their first birthdays as full term babies at their first birthdays. Maybe it's also because very few people know what day they were conceived, but most
Pediatricians use corrected age models, but most catch up by 12 months old. Significantly premature babies don't catch up until 2 or 3 years old sometimes though