Easter egg: The example is named dogcow, after a 90s Mac icon, designed by Susan Kare, which later became a small mascot: https://512pixels.net/dogcow/
Regarding .length: Effectly that is just the result of Unicode, there is no one-to-one equivalent between characters code points, the code units in an encoding and the resulting grapheme clusters. That is in effect a result of the complexity of the world's alphabets, including Emoji.
The other aspects of strings are also centered around things being of uncertain length, like how it's O(n) to take the nth character of a string, and how there are rather complicated objects involved in taking substrings. There's a lot more thought and resulting complexity than other languages' default strings. And yes a few languages use extended grapheme clusters, but I feel like emojis were the real motivation.