There's a reason for that.
Because it's not Norse mythology. The Norse were just one regional population that followed the same mythology as . . .
Well, yes, but the originals were Norse. The number of days and naming them after gods came from the Greeks (Sun, Moon, Ares, Hermes, Zeus, Aphrodite, and Cronos), which the Romans then adopted, but switched to their gods' names (Sun, Moon, Mars, Mercury, Jove, Venus, Saturn), which was adopted by the English, but switched to their Old English names of gods (except for Saturn, which we keep as Saturday), which were from the Germanic tribes, but they got them from the Norse tribes.