Well they do have the budget for it, but
(a) what you just said
(b) Avatar is an originally English series with an established cast of VAs, so chances are you won't be able to find people who can convincingly replicate those voices
(c) porn game voice acting is seen as "cringy" by the majority of people the same way anime dubs are, and gets disabled
(d) going back and recording, editing and adding a voice clip for every line of dialogue in the game takes a non-trivial amount of effort, which people would view as time better spent on adding more content, for the reasons listed above
Just give the "AI" meme about 5-10 more years to mature, and you'll be able to use an algorithm to read out the voice lines for you using the original characters' voices.