Because, with VOIP there is no power down the line. We live in the middle of nowhere, with no reliable mobile signal, if our power goes down and the optical router is off, we have no phone.
I was going to post up a thread about the digital switch over as I'm currently looking into it for my elderly Mum.
So yes, VOIP will need power to work but also if there's an outage at your ISP that can also mean your phone is down. Not that uncommon.
My Mum has a fall alarm that uses the land line to call for help. There's no mobile phone reception in her house on any network, so she needs the landline to call for help occasionally. It's not unusual to have power cuts and some last for several hours, probably the most likely time she needs the landline to work and I gather the current backup options will only last for an hour.
Not good.
As an aside, our last long power cut was over 24 hours and both our local phone masts on two different networks failed. Funnily enough our analogue landline stayed working.