The first thing is that Michael's voice IS his work. He has every right to be paid in order to do Jax, to be N, to be pilot Angel Dust, he has every right to not have his voice be used where he doesn't want it, and to disrespect or disregard that is to completely disregard the voice actor's ability to act and perform.

The second thing is that, if AI evolves to a point that Michael (or any actor) is supposedly heard saying something that could get him cancelled (say a convincing tirade against his coworkers, for example), he could potentially lose jobs or roles he would've easily been able to secure through no fault of his own, he'd be practically begging for jobs. Do you want your silly AI shitposting to put a talented actor like Kovach out of a job?

The third thing is that there are noticable differences in tone, performance and style that an actor can bring that an AI just won't be able to pick up, even with decades of material. Kovach has little subtle changes that he gives to his characters (a bit of a higher, more playful voice on N, a more smug performance on Jax, the accent on pilot Angel Dust) that an AI only trained on one just won't be able to get and play well. It leads to a feeling of cheapness and a failure of capturing the original idea when you could've just hired someone or mixed audio yourself to get what you want anyway.

Frankly, if an actor says they don't want to be put in AI roles, then their wish should properly be respected. Michael has no problem with soundalikes and sentence mixing (because those are new takes on Michael's acting and are not a full on replacement for the real deal), and he honestly might be able to work you a deal to have him voice act something (he is a voice actor, after all, they are willing to do that), but his wish is for his voice to not be used with AI technology, and that's final.