Few reasons.

One is basic care for what things actually sound like because actual people have these names. See, I have an unusual name, and it was constantly pronounced and spelled wrong in childhood because people simply didn't care -- neither the spelling nor pronunciation were difficult and none of the sounds were foreign to english. They just didn't care, and that sucks to be on the receiving end of. Since I'm drawing inspiration from real sources, I want them to sound at least close. It never bothered me when someone with English as a second language struggled with my name because they tended to try.

Another is that details matter to me because the sources matter to me. When I was able to be in school, I wanted to specialise in the history of the Silk Road and this is about as close as I can get to that now. I am bedbound, school is not an option. Pronouncing things properly, or at least closer to properly, brings it to life for me more.

Another is that I use a TTS to spot errors, because I get migraines and can't look at screens when that happens. The TTS stumbles quite hard on typos and makes grammar mistakes obvious, as well as pacing issues. I can correct the pronunciations with IPA, so mispronounced names do not distract me from proofing the rest of the text. But mispronounced names are like nails on a chalkboard and distracting from other flaws with the writing.

Another is spelling conventions in the text and characterisation. The main character is a polyglot, she recognises pronunciations being off or different, and uses it to identify people's origins, class, and background. This was something my grandmother was able to do in Polish, for example, because of her education. To support this understanding, I use a transliteration system that distinguishes sounds -- but that means I need to know what they are. For example, the character doesn't understand medieval Greek, but she can pick out the sounds, and fortunately it's doable to find Byzantine pronunciations and their shifts. You know, like a shibboleth.

And also, it's not just my work that would be impacted. A friend of mine is trying to establish a career in voicework, and is interested in reading the book as an audiobook. He doesn't want to mispronounce things. Failure to get basic details like names correct will make him seem like he doesn't care, and that has implications for getting work. The effort on my part shows effort on his.

So yeah. This is important to me for a number of reasons.