Because "you" want to sell something to the average Joe or grandma, who wants to just plug it to the power and to the garage, and be done.

There are many cases where the cell phone won't be in the same WiFi of the appliance and on those cases you can:

1 - use some intermediate service: cell phone app <--> cloud <--> appliance

2 - Cell phone app <--> VPN to your WiFi network <--> appliance

3 - Cell phone app <--> router/modem with open ports or redirect ports ou DMZ <--> appliance.

The solution "1" is under your (seller) control, so that it's easy to provide.

Solutions 2 and 3 (and perhaps some others) need intervention from the customer, in some complicated settings. Use it and you'll surely limit your market.

The first (second?) company that creates a protocol fro their modem/router that allows a simple configuration of IoT devices to your cell will make money. You know, like WPS was created to make it easy to connect new devices to your WiFi.