It's because people like choice and control, and "5.2" vs "5.2 thinking" is confusing. Making them "5.2 instant" and "5.2 thinking" is less confusing to more people. Their competitors already do this (Gemini 3 Fast & Gemini 3 Thinking).

ChatGPT 5.2 Intuitive

ChatGPT 5.2 Ponderous

“I had this dream the other night…” – https://www.youtube.com/watch?v=6gYIbMwswKM