Couple of reasons for me:
1 - way too lazy to wash, dry and iron all that fabric
2 - never had a problem with it shrinking
3 - it's going to get full of thread, fur, feathers, coffee (once) while I'm sewing, so why bother LOL
4 - I ALWAYS wash and dry every quilt once I have completed it. And I don't put it on delicate ... I put it in the machine as if a "non-quilting normal" person would ... regular load, and 30 minutes in the dryer. If it falls apart, it wasn't meant to be LOL I've never had a problem with any of them.