Out of curiosity, was the structure of the tree created based on letter frequencies similar to trees within a Huffman Encoder? If so, shouldn't O, I, and S be higher up the tree?

In real morse messages, you aren't always sending full words, most of the time you are sending prosigns (abbreviations) - Think like early SMS chat. "DE" means this is, you hand over with a "K", "SK" means you are going offline, "HIHI" is like LOL, "XYL" means significant other, etc...

Things like Q get used a lot in Morse for example (look up Q-Codes) compared to real English.

So, traditional letter frequencies are not applicable (Also because the code itself is language-agnostic)

Using a tree like this to decodee also sucks from a speed perspective. You need to learn the sounds /not/ count the dits/dahs if you want to get a good word per minute.

source: radio ham who knows morse and can do 20+ wpm.

A lot of weird stuff in morse and things also comes from older comms methods like Semaphore - The reason we have M in Scottish callsigns for ham radio is because M is the saltire (Scottish flag) symbol in Sempahore, for example.

Morse code is ~180 years old, and was developed over a period of years. Figure that awareness of letter frequencies, Huffman encoding, etc. was pretty limited back then. And cruft accumulated in the developing standard. And a number of sub-optimal choices might have been made to address "very similar encoding of words with very different meanings" issues. And...