People buy the cheapest shit cable modem/router they can buy and use it until it physically dies or rent a very basic unit for a large space. Because they are unwilling to buy or rent sufficient hardware there is going to be spaces in the house where a temporary drop or dip that is going to turn into a roam on an adjacent network.
Fortunately my carrier doesn’t do this, but having to manually select my own Wi-Fi every time I come home (so that I can reach local devices) sounds extremely annoying.
I‘d hope that the iPhone would at least periodically rescan for higher priority networks.