That’s not a coincidence! Naturally, it’s less work to transmit shorter sequences of dots and dashes, so we try to use up all the shorter sequences first. Basically, this means that we fill in all the branches at one level of this tree before moving onto the next. The result is a perfectly balanced decoding tree.
The placement of the letters is also far from arbitrary. Here are all the letters in English ordered from most common to least common:
ETAOINSRHLDCUMFPGWYBVKXJQZ
Notice something? The shortest morse code sequences were assigned to the most common letters. This makes the common letters easier to remember, and makes messages as short as possible in the average case.
Numbers are sort of an exception to this. All numerical symbols are encoded with 5 dots and dashes. But there’s a pretty clear pattern to these as well.
So if the listener hears a series of 5 dots and dashes, they immediately know it’s a number. To decode it, they count the number of dashes. If the dashes came before the dots, the number is 5 + the number of dashes. Otherwise, the number is 5 - the number of dashes.