To be fair, both Commander Shepard and Geralt of Rivia have way more character and a better story than the Sole Survivor.
The player has no real investment in the SS as this is a standalone story, and yet adding in a voiced protagonist only deepens the rift for player investment because there is indeed "some" personality, but it's utterly throwaway as they'll only ever feature here, in Fallout 4.
Fallout 4 is the story of the Commonwealth and your role in the events that transpire. You are but one character in one specific area in the post-apocalypse, and there are many more like you. The overarching theme of Fallout is one of the world after the bombs fell, and each game is a localised instance of an individual rising up to change things. The PC is both an avatar and a vehicle for the experience and that's why people feel estranged when they gave your character a personality.
Personally, I find Fallout less immersive than The Elder Scrolls, but I don't think it's to do with it's RPG aspects per se. I believe the world of Tamriel is so rich and beautifully crafted that it is the real star of that franchise, and is what keeps many players (including myself) both immersed and invested in the experience.
Fallout, on the other hand, has a ruined, retro-futuristic 1950s America theme, and in all honesty, the world itself is getting old and tired. Not to the point where people have lost interest, but certainly enough to result in a loss of genuine care for the world itself. This is part of the reason why I've maintained the stance that we need a change of scenery in Fallout. Seriously, ANY other country will do, it doesn't matter which.
Furthermore, the world of Fallout isn't even that hard to imagine. Tamriel is pure fantasy, but "ruined America" is all too easy to envisage imo.
My point being that the PC has never been a Bethesda forté, but the world in Fallout is wearing thin. Throughout TES we've had myriads of different monsters and factions and eras. Fallout has the Super Mutants, the Brotherhood of Steel, the Enclave, Feral Ghouls, Mirelurks, Yao Guai, Raiders, Deathclaws, Radscorpions, Bloatflies and various robots; and the worst part is, all they do is cycle the same characters/creatures/facets and just add in either "one off" factions or characters, or a comparative handful of new enemy types.
And do you know what else? The whole ignoring of New Vegas in Fallout 4 really hurts the game too. In every TES game, the world feels connected to whatever happened before and it really conveys that sense of continuation with the lore. Taking Fallout 3 as the rebirth of the franchise (and scant few references to anything that came before) they establish the fundamentals. New Vegas built upon that with more factions and made a lot of content of what happened, in the games prior to FO3, both digestible and current to the franchise. Fallout 4 distanced itself from that, added in a voiced protagonist (a character that will last for one game, and be a footnote in future games) and gave a couple of nods to FO3. There's no real world-building for the franchise between games, everything is very instanced and consequently, forgettable.
The cast doesn't change much, the stage props are falling to bits and the audience is tired of saying "He's behind you!" in morally ambiguous terms.
That's how I see it anyway.
I still enjoy the series and Fallout 4 though, and I have no regrets whatsoever over my purchase of both the game and the season pass.