gpt-image-1.5 works decently for generating images compared to old Sora, but you pay per generation. It's possible that monthly flat rates were too much of a loss leader for OpenAI. I imagine the server side cost for generating video for Sora 2 is much higher as well.

You also have access to gpt-image-1.5 in the regular ChatGPT interface if you pay for a flat subscription - though I don't know how many images it limits you to per month.