I experienced the same when my husband died.
The assumption (I think) is that if you estimate that the annual mileage will be say 10,000, then two drivers will do 5000 each. One may be a better driver than the other and that will effectively halve the risk of a claim.
I was advised, by the insurance company to nominate someone, anyone who agrees to it and is perceived to be a good risk, to be the second driver even though they may never use the car.
I've had a randomly nominated friend on my policy ever since. Each year when I renew, and it's been the same when I have changed insurers, all they ask is whether the named person has made any claims. In all these years, no-one has asked me how we share the mileage.
It's a silly rule and yet another example of how single people are militated against.