I don't think it's that deep, it's just how we say it over here. People do sometimes say it the other way, too, it's just less common. If someone is just talking about a date in the same month we're currently in we usually just say the number without the month (the 12th). It's interesting to hear it really does seem to mimic the way people say it out loud in every case so far in these replies!

Well if we're talking day to day life, when I ask someone what day it is, they're not going to say "It's December 12th 2023". They're just gonna say "The 12th" because it's true the month doesn't really matter day to day.

I think the only time people use the full date is on like official forms where the month is more relevant. Also this is America, we don't care about average people's day to day, just businesses and money and cheeseburgers