So all of you are riding without the seat?
Kidding. But most bikes are shipped with the saddle and seatpost assembled together, but not installed.
I have assembled thousands of bikes, and I teach others how to build bikes as well. My take is that while they can be made ridable with minimal assembly, they can always use some additional adjustments. Many parts lack grease (or lube, assembly paste, anti-sieze, thread-lock). Non-sealed bearings are almost universally delivered too tight; grinding bearings equals premature wear. If a company provides a pedal wrench, hopefully it is long enough to make tightening them easy. I cannot tell you how many times people have ruined their bike's crank because the pedals were not installed with enough torque and the pedals fell off. Crank arms should always be checked for proper torque as well. Wheel spoke tension is normally okay, and lateral true as well; sometimes radial true is poor. Regardless of true, wheels need stress relieving; without it, premature spoke breakage is more likely. Rear derailleurs are usually okay for the limit screws (although most the rear cable housing is too short), front derailleurs is more hit-or-miss. Discs brakes are usually delivered fine, but rim brake shoe alignment (especially linear-pull brakes) is often lousy. Both brakes and gears always need cable pre-stressing; otherwise they go out of adjustment quickly.