Why the heck do people keep breeding spider morphs?

[Neal]

The majority's reason being that the genetic defect associated with spiders doesn't interfere with their ability to eat and thrive as a normal snake, at least in a captive environment as pets. It just gives them a lopsided motion when they're moving about or they rest in funny positions.

This ^^.

Spiders do amazing things to some morphs.

As red said, perhaps you consider it a defect, but it's just part of a snake that makes them special. They can still live a full and healthy life as a pet.