Perhaps it stems from Steve Jobs's philosophy on quality:
“When you’re a carpenter making a beautiful chest of drawers, you’re not going to use a piece of plywood on the back, even though it faces the wall and nobody will ever see it. You’ll know it’s there, so you’re going to use a beautiful piece of wood on the back. For you to sleep well at night, the aesthetic, the quality, has to be carried all the way through.”
As for Instagram, Pinterest or SnapChat: they won the lottery. They were in the right place at the right time, and when that happens, you could design a piece of junk and still experience massive success, a least temporarily, as long as you have the one feature that matters (re: MySpace, MS-DOS).