Because if the client specifically requests GPT-3.5, but is silently being served something else instead, the client will rely on having GPT-3.5 capabilities without them actually being available, which is a recipe for breakage.

You do understand that the client will be written by the same people setting up the inference server?

Because it's lying to the client?

And why is that bad?

Your mindset would mean that Windows would have next to no backwards compatibility, for instance.