Good context and explanation. Thanks. After reading that, I’m less sure my reasons apply as much to your particular situation, but I’ll throw them out there anyway:

1. They assume you don’t just want the answer but why that is the answer. For me, I tend the learn and remember better if I understand why X is correct. If I’m just told X, I’m more likely to forget that answer later. If I have the context around the answer, I understand it better and can recall it better. Similarly, they may think you have (or will have) similar questions, so are showing you how to find the answer to those related questions.

2. They don’t realize your question is precise/pointed and think you will have follow on questions, so are answering the potential related questions.

3. if the question doesn’t have one right answer or they think their answer is right but could be wrong, they are providing background sort of to say “This is how I came to this conclusion, but you may come to a different one or there may be alternatives.”

4. they just like to hear themselves talk and are happy to have a break from their work.

Aside: when I said “they assume you don’t just want the answer…” or similar statements, I don’t mean that they are literally stopping and thinking about whether or not that is what you want. It’s probably subconscious and their default way of answering questions. Understandably, this leads to your frustration: even though you are giving a clear, well thought out question, they aren’t stopping and thinking about why you asked the question that way. Instead they are answering in their default mode.