How? They have no idea how each couples' driving is split and they may not fit the average at all. As it happens, in this house, my wife drives a handful of times per year and only locally and the vast majority of my driving is in her car, yet the insurance has her named first - no, we are not scamming the system, it is a Motability car and she is required to be first named, even if she never drove it.
She is also named as a second driver on my car, just in case, but has never driven it.
There are reasons for us still having two cars - I can be working away and she may suddenly need a car; I can only drive the Motability car for her benefit, not my own, so work, shopping, etc. is fine; but not me just going alone for a night out with friends; her EV hasn't the same range, ability to tow or space to carry things inside as my Zafira and isn't rated for a roof load.