I think this is because most companies do pager duty wrong. I highly recommend the google SRE book[0] (notes here[1], chapter 11 covers oncall/pager). One thing mentioned in this book is compensation for being oncall. At Google we get fairly decent pay compensation for holding the pager, enough where it can incentivize people to be on the rotation.
(I'm a software engineer at google who is oncall at this moment)
(I'm a software engineer at google who is oncall at this moment)
[0] http://shop.oreilly.com/product/0636920041528.do
[1] http://danluu.com/google-sre-book/