The reason is because a voiced protagonist, over time, inevitably develops a personality of their own owing to the way the VA voices them. Good examples include Mass Effect's Shepard, or Dragon Age 2's Hawke; I never really felt like those characters were MY character; instead, there basically is already a general idea of who Shepard/Hawke is going to be
Fascinating how different impressions can be. Mass Effect is exactly the example I personally would use for why a voiced protagonist can be great even without having 10 voices to pick from. I guess your opinion is similar to some people's aversion to playing Larian's origin characters instead of fully customized ones... the wish or need to leave your own mark on the protagonist.
The far too simplified dialogue prompts in ME or DA:O are a different problem of course, as they often gave no impression of how the actual answer is going to "sound". That's not going to be the case in BG3.
And yes, there are games with mute protagonists,
but the huge difference is that they don't look so freaking realistic. NWN and DA:O are both quite old, the graphics show it, and you usually see the back of your character when they talk.
Then go look at BG3. Here's a link to an early dialogue scene where you can recruit Shadowheart (I deliberately picked that one because it's very non-spoilery. In romance dialogues the effect is even more obvious but I figured some people wouldn't want to see those yet). You can start watching at 1:00 to see what I mean.
Fully custom main character. Multiple dialogue options and you see the whole text.
And yet, seeing the level of realistic visual details, hearing Shadowheart speak (who has a great voice) and then looking into the face of your mute, fidgeting protagonist, clicking on a mute dialogue option and then seeing and hearing Shadowheart's full response again, the protagonist just looks and feels so incredibly awkward, very immersion-breaking to me, much worse than having a limited number of voices.
I guess it all depends on person taste, how much importance and weight the "full custom" thing has for someone, vs the odd feeling of watching a movie dialogue scene where one audio sequence is missing or broken.
And especially in romance dialogues that feels sooooo weird to me.
When we had those discussions in theory only, I remember saying that while I preferred a voiced protagonist, I wouldn't mind too much not having it, and couldn't understand Antimatter's adamant position on this. Until I decided to think "forget spoilers, it's so early in the game" and watched a few clips of camp interactions, especially the ones leading to romance interaction, and then I totally got it.
So we just have different ideas of what immersion means to us, but if they decide to voice the origin characters only, which are already there as NPCs otherwise, then it would be a reasonable compromise, because people who wish for full customization wouldn't pick one of them anyway. Although, if Divinity: Original Sin 2 is any indication, it might be possible to fully adjust the looks and maybe even class of an origin character protagonist.