The simple answer is: a guitar pickup isn't anything like a loudspeaker, and a loudspeaker isn't anything like a guitar pickup.
They are two entirely different things and this should be pretty obvious to anyone who uses them both.
Why anyone would compare the two is puzzling to me.
As I say basic non technical answer ............... and hopefully right 
Basically correct, although a microphone converts sound waves to electrical signals, whist a pickup converts electromagnetic waves into electrical signals.
The electromagnetic waves occur when you move an electrical wire through a magnetic field. in this respect, a guitar pickup has more in common with an electrical generator (converting motion to electricity) than a microphone or speaker.
Although they use many of the same fundamental principals, they are completely different things. To understand the differences full requires some understanding of electronics.
As I say, it puzzles me why anyone would compare the two.
Cheers,
Keith