My speculation is that frontier models are MoE, and they just have some number of experts for coding.

OpenAI do have codex models, which are half the price. I haven't used them enough to comment on the quality though.

I remember them saying a few years ago that, they didn't think it was worth specializing models for code, because their general purpose models kept beating them. I guess they changed their mind? Since they did start making codex models again.