Okolonan
First time out of the vault
They are simply approaching the game in a different way. Being surprised by what your character says is a flaw because it can undermine the character you are roleplaying or can affect the world in a way that you didn't want. If someone is playing Fallout 4 as an open-world action game with character customization and a dialogue wheel (not a role-playing game per se), then being surprised by dialogue obviously isn't an issue for them since they either aren't roleplaying or just aren't very invested in that aspect of the game.
Also, just in terms of mechanics, it would be annoying to hear your character speak a line right after reading it in your head and hearing it in your internal voice. That would also ruin the delivery since you already knew what the characters were going to say, having just read it a second before. I don't know if Bethesda decided on the dialogue wheel or the voiced protagonist first, but I think a voiced protagonist pushes the game toward this feature where you get a description of the line that will be said rather than the line itself.
Also, just in terms of mechanics, it would be annoying to hear your character speak a line right after reading it in your head and hearing it in your internal voice. That would also ruin the delivery since you already knew what the characters were going to say, having just read it a second before. I don't know if Bethesda decided on the dialogue wheel or the voiced protagonist first, but I think a voiced protagonist pushes the game toward this feature where you get a description of the line that will be said rather than the line itself.