It's the same way how every AI generated poster looks exactly the same. As if there is a single underlying prompt that describes the template of the poster/long-form article, and it does not dare deviate from that.

Isn't there? Like everyone using $MODEL is starting from the same base system-prompt. Then our user input is a small bit on top of that core mode. Like what would happen if everyone asked Mikey to paint their ceiling - they'd all be similar and therefore boring.