For the same sort of reasons there are (generally) 12 months in a year and there are 7 days on a calendar, and for the same reason that “John” is a name, and why London is placed where it is, and etc?
Because some dudes decided some stuff, and some other dudes decided some stuff influenced like that, and so on. And some stuff got changed, and some stuff was inconvenient to change or there was no real reason to change it.
The year is ironic in the exact context you quoted I guess. But the days of the week and many months were named for other mythologies.