I wonder if there’s a technical reason making an on-screen keyboard difficult? For sure, typing on my Kindle is pretty slow and imprecise, and wouldn’t come close to acceptable on a phone.

Also, there are phone-shaped e-readers if that’s your bent; check out the Boox Palma.

Typing on the Palma's screen sucks due to the input delay. At least if you have fat fingers like me and prone to making typos on touchscreen devices.

Typing on the Palma with a BT keyboard, however, is an absolute joy.

my guess - e ink refresh rates / ghosting suck and it would be hellish hard to get an on screen keyboard with an e ink display to get anywhere the typing speed a modern touchscreen can deliver.