After code is written it goes through a compilation step that makes it easier for machines to read and harder for humans to understand.

Think of the index of a reference book. Where every pertinent word is listed along with a page number. Imagine you wanted to read a complicated and meaniful book like Tolstoy or Kafka and really understand it. Walk away with a sense of exactly what it meant. To gain that understanding you only have an index and you had to reconstruct the book's meaning from there. Technically you can, but it is massively more difficult.

Edit: A more accurate analogy would be that you aren't even given an index of words, but an index of phonetic sounds that you have to piece together to create words.

It would be substantially more expensive to audit the assembly and verify its security