Because those don’t boot into a fixed interactive programming environment with a BASIC-like language and REPL to easily do simple things on the same screen and using the same keyboard you also use for programming. Your proposed setup has more complexity and is less intuitive for a learner.

For starters, there is way more friction both in buying hardware and waiting for it to arrive and developing on real hardware in general.

I agree however that it's super cool to have real hardware to run this on.

Because those require you to get to grips with “a tiny i2c/oled display, wires and set of basic switches” when you’re interested in coding, not hardware.

Because moving a sprite is much more exhilarating than blinking an LED.