Because we guitarists enjoy the wild goose chase that is the never-ending quest for ultimate tone, and thus need to try as many pickups as possible.
But on a more serious note - it's because woods sound very different, and pickups will sound very different depending on what guitar they're in. It's a pain to take the strings off, de-solder a pickup, solder in a new pickup, and put on the strings - only to discover that the pickup doesn't sound any good. Being able to swap them out in mater of seconds is an all-around improvement.
But on a more serious note - it's because woods sound very different, and pickups will sound very different depending on what guitar they're in. It's a pain to take the strings off, de-solder a pickup, solder in a new pickup, and put on the strings - only to discover that the pickup doesn't sound any good. Being able to swap them out in mater of seconds is an all-around improvement.