You don't need an extra document to define each term as it is expected that others in the field will understand the language used.

For lawyers, it's the opposite, actually. Lawyers are overly cautious and choose to explicitly define terms themselves, all the time. If they can reference a definition already in a specific law, great. But they'll go ahead and explicitly make that link, instead of relying on the reader to assume they know which law to look up.

So any serious contract tends to use pages and pages of definitions at the beginning.

Imagine programmers being reluctant to use other people's libraries, but using the same function and variable names with slightly different actual meanings/purposes depending on the program. That's what legal drafting is like.