Typing when you need it gives you more freedom over a toggle. You can choose to type some parts of the code while leaving other parts untyped.

For example, if I'm writing a quick and simple Python script I may forgo typing, but when iterating on it I'd go back and add the types I need.

why is there no switch to enable type checking at runtime?

Have you got problems this would solve? I've done a lot of type annotated Python at scale and I can't think of an example.

Edit: given nobody in their right mind allows code that's not checker clean.

You said it's not runtime type checking but then switched to "strict type checking" - those aren't the same. Other person has it right, it has runtime type checking. The type checking happens when running your code. I don't think that's particularly useful, it's pretty much sugar on top of what would throw an error anyway.

it lacks clear and enforced type restrictions which help with clear code contracts

Not anymore! Gradual typing is supported by the core language and pyright is a fantastic incremental type checker that you can use both in your editor and in CI.