Hmmm... that’s is a tricky subject, but I wouldn’t categorise it as good/bad writing. OP seems to suggest that Larian writing team is fully capable of writing expressive lines - they just don’t do so for PC.
I will ignore heavily pre-defined playable characters (Shepard, Geralt, Hawke) as those are on completely other part of the spectrum. Let’s stick to more “open” RPGs - Baldur’s Gate1&2, Tyranny, Pillars1&2, Fallout. While those characters aren’t as defined as Shepard/Geralt/Hawke they are still quite defined - you are a Gorion’s ward/child of Bhaal, Fatebinder, Watcher, vault dweller. Within those rolls you get a range of who you get to be. While it’s more granular approach, then let’s say Hawke diplomatic/aggressive/humorous, but in many ways it is the same thing.
I think Larian aims for a much bigger player freedom. Tadpole seems more like a driving plot gizmo, rather then definition of who we are as a character. Various PCs also don’t share background, history, geographical knowledge, world knowledge. No matter who you are in BGs, you always grew up in Candlekeep, you always had dark destiny ahead of you, you loose your mentor, and world outside Candlekeep is alien to you. That’s a lot of stuff to write about, which will be true of every character you create.
I don’t see how Larian could write a more expressive PC lines, without heavily restricting who player characters are. And while companion lines might be well written, keep in mind those varied and expressive companions will also need to use the same lines written for PC, if they are picked as origin. PC isn’t another character, whom we can mole to some extend - he is a set of actions, abilities and knowledge available to us, based on our “tags”.
I don’t know if it’s an effective approach, but it is what it is.
