Voice acting is nice, but it is also quite costly to implement. Arranging recording studios and a variety of voice actors is far more work than simply making text.
This is particularly important for Elite, as they don't seem to have the budget to make more than a dozen written lines for NPCs. If they can't afford to have an intern spend an afternoon throwing together a hundred more written lines, how can they afford to hire dozens of voice actors and the required recording studios?
There's also the issue of localisation. It's all well and good for us, who all presumably speak English, but for foreign language users they either have to rely on subtitled recordings or they simply miss content, unless FD are willing to dub the spoken parts into other languages too. However, they could simply run with the "multiple language" issue and actually have voice recordings in a huge variety of different languages (or even weird combinations of languages, such as the vocabulary of one but the sentence structures of another because of linguistic drift), so there's a real possibility that you will find a hidden recording that you cannot understand.
Not to say that it can't be done, some games have truly gargantuan amounts of recorded voice lines. Of note is that Divinity: Original Sin 2 has over 1 million words of recorded dialogue (or 74,000 lines if you prefer) and that was made by a small-medium studio with a fraction of the resources of what FD has at its disposal, so if FD wanted to they could quite easily put 50,000+ lines of spoken dialogue.
That being said, I'd rather have 100,000 written lines that add variety than 100 spoken lines for select circumstances. Even better, would be for communications to be more procedural, even if they are simply based on certain templates and sentence structures with minor procedural alterations. If there's a 100 initial templates for a pirate identifying you for a mission, with 6 replaceable words that each have 20 different alternatives then, with relatively little work, we would have 6.4 billion different combinations. This could be expanded further with certain factions using certain terms, linguistic roots or slang rather than just making it completely random.