The issue is the scale of the improvements. GPT-3.5 Instruct was an utterly massive leap over everything that came before it. GPT-4 was a very big jump over that. Everything since has seemed incremental. Yes we got multimodal but that was part of GPT-4, they just didn't release it initially, and up until very recently it mostly handed off to another model. Yes we got reasoning models, but people had been using CoT for awhile so it was just a matter of time before RL got used to train it into models. Witness the continual delays of GPT-5 and the back and forth on whether it will be its own model or just a router model that picks the best existing model to hand a prompt off to.