I noticed that quite a number of open-ended RPGs (those on the PC) like Star Wars: Knights of the Old Republic series, Morrowind, Baldur's Gate series and Oblivion to name a few seem to be following this trend of putting you (the player) in the role of a main character who is mute and customisable (be it facial and physical features).
My question is, why are these customisable but mute main characters seem to be so popular in most PC RPGs compared to set main characters that can talk and have his/her own personality? Isn't having an interesting main character as important as interesting team members or NPCs in any RPGs, let alone in any game?

To me, an interesting main character in any game is what drives the story forward.
I mean, doesn't anyone find these mutes particularly irksome when it comes to conversations, be it text or voice-acted? Whenever I play these types of main characters, I get the feeling that I'm playing a blank slate that seem to be very detached from the conversations of others and what goes on around him/her (or more like a spectator).
Compared to playing a main character that have a personality and can even talk, I prefer this type of main character (the one with the personality and can talk) as the player will feel like you're playing a living and interesting person rather than a blank slate who couldn't say anything for peanuts. This also will give me (the player) an incentive to know more about them and to progress further into the story with this particular main character as opposed to the mute one.