The flip side is that the other car is parked up in the same place a lot, so if someone wanted to steal it, it’s much easier to find. Also doesn’t decrease the risk of vandalism or someone else crashing into the car that’s parked up whilst you’re out driving the other one - indeed statistically hit and runs are much more common with unattended vehicles, so actually a higher risk of having to pay out should that happen.
What is is daft is that insurance underwriters will use both arguments to increase your premiums, but then that’s how they reduce their own risks and make their money.