Why do many/most (Android) phones have the power button opposite the volume buttons?

I have just bought a Nexus 5 phone. I love the phone, but like my previous two phones it gives me grief when turning the thing on or off. The problem is that the power button is almost directly opposite the volume rocker buttons, and so when I hold the phone in one hand my index finger naturally wants to press the volume down button when my thumb is on the power button. The phone normally turns on ok, but turning it off often results in a screenshot being taken instead. I have to hold it two handed or be careful of how I hold it. Often the buttons are offset slightly (as in the Nexus) but this doesn't seem to help and might actually be worse.

Why do designers persist with this? I realise the iPhone has the button at the top and also some Android phones (usually older ones, and this seems to have gone out of favour) but personally I don't like those ergonomics either; I don't see any 'natural' way of holding the phone when turning it on with a topmost button.

I'm coming from the perspective of a right-hander, but I imagine left-handers have it even worse.

Perhaps two buttons, one on either side of the phone and directly opposite, say about half way down and which have to be pressed at the same time, would be more natural. You would just pick up the phone and give it a squeeze. Perhaps not even buttons but just a depressable 'area' (or hidden buttons). This would also be more likely to avoid accidental turn-on's, IMO. That idea was off the top of my head, I'm sure there are better ones. Edit: Forgot; the above would also cater for left-handers.

Added: Here's others with the same problem Of course not all think it's an issue :)