because they keep these models loaded, and they can't just arbitrarily load up whatever models you want.
but it's more likely just a business case: they need you buying higher tier model output. They know whose doing what, so someone needs their 3Q bonus.
I was going to reply that Anthropic, which supposedly is the most capacity constrained of the leading AI labs, still provides access to models as old as Opus 3.
But then I realized Opus 3 is an outlier, and Anthropic has removed access to relatively more recent models. https://platform.claude.com/docs/en/about-claude/model-depre...
I wonder what the deal is with Opus 3.