I'm probably a little bit off on my assumptions, but...
With stuff like this, time literally
is money. There are people making this game, and they all have to get paid. Like in the movie business and similar media - this generally comes out of capital that is received beforehand by investors for an expected return on investment. Looked at from that view, the "deadline" is simply when that money runs out, and there's no longer any money to pay these guys to work on the game any longer.
Obviously, that's on overly-simplistic view of the business. There are companies that have a reputation for delaying release of a game until it's "ready." (Blizzard comes to mind, for example.) But generally, it seems like most companies have to struggle more in balancing between monetary concerns versus quality.
(That being said, that's really no different than any other media. A writer can spend ten years writing one novel - but they're not going to be making a living doing that, and no publisher is going to advance them enough money to be able to do that independently. Even the most "indie" of movies has to get finished at some point. Coming from a classical art background myself - and taking "videogames as art" as a given - all art stems from striking a balance between practicality, perfection, and creativity. I can't remember who said it, but there's a famous quote something along the lines of "no great painting is ever finished - only abandoned."
That's likely true of videogame design, as well. You're only ever going to be able to do so much. And the end result is always going to be better if you set a goal that exceeds your practical limits, and pare back from there. If you end up creating something that's actually exactly as good as you'd intended - then you probably weren't really trying hard enough.
)