Smartphone software like messaging or notes is built with imprecision in mind. I don't know about you, but I use the swype method (moving a finger continuously from key to key and lifting at the end of the word) and when writing a message I still spend like 50% of the time correcting mistakes from the virtual keyboard.
Because these are usually short messages, you'd instead likely be hunting and pecking the virtual keys, but without the tactile feedback of a real keyboard. For games that are all about text, it's a preference thing.
Plus a phone comes with a ton of distractions that a dedicated device without notifications doesn't. Sure, you could turn on do not disturb while playing a text game but that feels a little intense. Don't bother me, it's Zork time!!
And if you really think about it, your question is basically like asking why anyone would need a Nintendo Switch when mobile games have virtual controls.