I don't think it's that anyone wants the interface to be chat. I think it's that the underlying technology is inherently words in and words out. It's similar to how devices with capacitive screens are most naturally going to support tapping, dragging, and pinching.
My personal hot take is that the product people (engineers too probably) at these companies are just straight up lazy. Yes, a new UI paradigm is hard, but it's been painfully obvious that chat just absolutely sucks. It's also obvious that some DSL-ish thing is possible, something that just does token juggling and the end-user sees some UI behavior.
I know this all sounds abstract. I've been mulling over it for the past year and it's very hard; and LLMs are super janky and inconsistent so it's 100% not trivial. So in some sense I understand why a lazy bottom-of-the-barrel "chat interface" has become the de facto standard.
It's funny to me that instead of moving existing tech to a word in word out paradigm, people are dropping millions getting the chat to work with the existing paradigm.
Probably inevitable, but seems like a lot of disruption could happen there.