When OpenAI went from GPT-3.5-Turbo to GPT-4 it seemed massive, and there were no other steps in-between. And nobody else had a meaningful competitive model out yet.

When GPT-5 came out, it wasn't going from GPT-4 to GPT-5. Since GPT-4 there has been: 4o, o1, o3, o3-mini, o4-mini, o4-mini-high, GPT-4.1, and GPT-4.5. And many other models (Llama, DeepSeek, Gemini, etc) from competitors have been released too.

We'll probably never experience a GPT-3.5 to GPT-4 jump again. If GPT-5 was the first reasoning model, it would have seemed like that kind of jump, but it wasn't the first of anything. It is trying to unify all of the kinds of models OpenAI has offered, into one model family.