Really, the only bikes/frames that are going to be shipped with a seatpost installed are the ones with integrated posts that need to be cut down. Otherwise, you need an even larger box to just accommodate the post and saddle, which really are probably the easiest things in the world to install. Even if it came installed, you'd still have to adjust the height.
In fact, I know people who have opted not to buy certain bikes because they had integrated posts, which would make traveling or shipping the bike a challenge.
As for manuals, most are pretty much boilerplate safety information. I've gotten a one sheet instruction for installing some proprietary things, like a headset, but that's about it. In one case, I found instructions to NOT put a spacer above the stem, as well as a note that using a non-bike manufacturer brand stem voided the warranty on the fork. But it seems in most cases that torque specs are printed on the part.