I would propose probably mostly has to do with the sheer amount of text.
Last I saw an official count, the game was a about 1.2 million word about 2.5 years ago.
And supposedly each update adds about 50-60 thousand words on average
So, even with some of that being reworked content, it has got to be pushing close to 2 million at this point.
An average novel is around 70-100 thousand words.
If a 20 book series was released across 8 years would you say that was a slow pace?
The thing is that a lot of people don't realize how much text is really here because they only see a fraction of it in a given play through. There is a lot of content that people never see because they choose to go down a different path, or the RGN doesn't prop it for them, or they don't like the character that content is based around, or it involves a kink they aren't into, or it doesn't match with their sexual preference.
I would guess that the average player probably actually sees maybe a 1/4 of the content of the game, maybe even less. Just look at the responses when a new update is released. The update will list like a dozen newly added scenes but there are always tons of people saying something along the lines of "No CharacterA/CharacterB content, not interested."
Decent. But.
The original Baldur's Gate 1 contains about 1 million words. This includes dialog, location descriptions, items, etc. The team faced a number of challenges, including the need to develop a new game engine, the Infinity Engine, which would allow for games with 3D graphics.
Development of Baldur's Gate began in 1995 and took approximately three years.
Development of Planescape: Torment began in 1997 and took about two years. The game was officially released on June 12th, 1999. The game contains about 1.5 million words, making it one of the most talked about RPGs of all time. The game's dialogue is complex and multi-layered, often presenting the player with difficult moral dilemmas.
Development of Disco Elysium began in 2015 and took about four years. The game was officially released on October 15th, 2019. The game contains about 2 million words.
Ultima VII: The Black Gate (1992): 1 million words
Ultima VII: Part Two: Serpent Isle (1993): 1 million words
Ultima Underworld: The Stygian Abyss (1992): 1 million words
Ultima Underworld II: Labyrinth of Worlds (1993): 1 million words 1 million words
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Here we have a visual novel. Visual Novel. What a lot of problems there were back then in the early days of making games. The use of 3d. Economy. Mathematics.
And now that there are both engines and instructions, there are still questions for me about how long it took.