Why are passenger and cargo airplane noses manufactured separately from fuselages and then put together?
A radome, or nose cone, is essentially a structural cover serving a different purpose than the fuselage. These are made separate because they a special material invisible to the radar mounted beneath them. They are different shapes or sizes depending on the customer specific component they cover (radar antennas) and can be easily removed or replaced. 

The radome serves two essential purposes. 1) It covers electronic equipment mounted to the forward bulkhead of the fuselage to protect it from weather, or from view. 2) It acts as an aerodynamic cover for the leading edge of the aircraft.

https://en.wikipedia.org/wiki/Nose_cone