Because an app on the phone sucks to use, has no tactile feedback, is difficult for older relatives who may baby/pet sit, is more difficult to set up, etc...

There is a reason autocorrect wasn't really needed on physical micro keyboards but is 100% necessary on touch screen keyboards.

But hey, if people would rather have another service to maintain, tap on a slab of glass with no/shit feedback, have 30 minute explanations with their relatives and have to act as tech support on a holiday just to turn the TV on, more power to them.