Because companies can't afford their systems to be down for 16 hours (or 2.5 days if things fail at 5 PM on Friday). If that happens, the company goes bust and then nobody has a job.
Sure, there are bad ways to do on call, but if it's done right then:
The on-call team are only notified for things which are actually a critical issue.
On-call time (and in ...
I believe you overestimate the size of the problem.
I've worked in this business for 35 years, for 9 companies on 2 continents and none of those companies have had an actual "on call" policy in practice. There were only 1 where I was ever presured to work extra hours for more than an evening or two, and that was for a company-survival-level ...
Phillips excellent answer addresses a core issue, mine is just a more general view.
The practice is an old one. Companies do it for many reasons and it's normally a win win situation.
The company gets it's needs met in an emergency and the employee makes more money.
But there are also underlying reasons. It gauges a persons work ethic if nothing else, staff ...