I think it's because you expect there to be eggs in the box, you just don't know how many. So you're not referring to their existence or presence in the box, but their number.
You could say that as well, but "det är" is more commonly used at least with egg boxes. I'm having a hard time trying to figure out why that is.
"Det finns tolv ägg i kylen." would be the correct translation of "There are twelve eggs in the fridge."
"Det är tolv ägg i kylen." would work as an answer if someone asked you "Hur många ägg har vi i kylen?" Otherwise I thing it would be a little more of the surprised finding e.g. someone put 12 eggs in there without you knowing. "Det är en elefant i kylen." would sound about as strange.
English has the word dozen for twelve of anything, especially eggs and baked goods (cookies, donuts, etc.) I wonder if it is unique to English and if so, I wonder why.
In Swedish you can say ett dussen and in German there is ein Dutzend. So it's not unique to English but it might be unique to Germanic languages...
In Spanish we say DOCE for twelve and DOCENA for dozen and DIEZ for ten and DECENA for ten units. The etymology of dozen comes from old French via Latin.The use of twelve as a base number, known as the duodecimal system (also as dozenal), originated in Mesopotamia.
That question still torments me every night before I sleep. Please let me know if you get an answer.
In English, we usually keep eggs in a very specific type of box called a carton. Is it the same in Swedish, or are these eggs just in some random box?