Cost!
The main reason for using WiFi at home is that it uses your home broadband connection to get to the internet instead of your phone's data plan. For many folks that costs a whole lot less - e.g. almost unlimited at no extra cost for home broadband, versus limited and expensive for phone data.
Away from home it'll depend on any costs to ride on someone else's WiFi, but I expect it'll still be cheaper than phone data on many plans.
This is the case for me, and I expect for lots of other folks too.
Biting the hand that feeds IT
