And yet, Obsidian had in-house workers who did the ost of the first game for free (out of the goodness of their hearts came great deeds, from cheerful glee). It doesn't have to cost, there are and will always be people who do the job just as good as paid or free.
Being a musician I mostly guess that VO is expensive due to my personal experience in music. First you need a place to record - studios and concert halls are expensive. For a musician a recording session is expected to be better paid than a live concert, because your work will be reused by someone without you profiting. Sure there are a lot of musicians but they spent many years training, often deal with large student loans and purchased instruments worth $10,000-$100,000. Playing below union wage is not in their best interest. In the recording you have musicians and a team of director and audio engineers who will help in making sure recorded material is high quality and later edit it.
Recording of 30min-1hour long, high quality demo reel in US usually costed me between $500-$1000 and that was the cost of a small studio with one audio engineer and a very basic ensemble (just a single violinist - therefore not much equipment in use, no balancing to do) and it covered couple hours of recording session, and cleaning up/editing audio (naturally I didn’t pay myself for performing). I would also want to point out that as it was proof of my abilities recording, there was no time consuming editing like putting different takes together, adjusting intonation etc. Sure, I usually recorded for around $120 but the difference in quality was drastically noticeable. while I can’t possibly predict how much a recording of VO or soundtrack for a game costs I expect it to not be a cheap process.
We are forgetting how common good voice actors and musicians are to find. The voice actor strike didn't harm games, except for the companies who refused to look outside of what was recommended to them.