> I don't get for why people think it isn't a friendly human computer interface

Because the human is not friendly and has near 0 margin for error.

> The way we've dealt with speaking is just learning to speak clearer,

No, the way we've dealt with that is also to understand less clear speech.

Doo u undrstnd mi? None of those words were even actually correct yet I imagine most people had a good guess at what I said.

An unfriendly human interface that acts like many CLIs:

Hello I'd like a burger

NOT UNDERSTOOD I'D

Hello, can I have a burger?

NOT UNDERSTOOD CAN

Hello, burger?

ERROR ITEM BURGER NOT FOUND

Hello, cheese burger please

Good evening sir, thank you for placing your order.