Because what you're asking for precludes the concept of mathematical guarantees. I'm not taking your question at face value, because you could be asking why call site code should break when the type signature generalises (which is a useful thing), but that's not what you're asking.

It seems you're asking for code to be both null safe and not null safe simultaneously.

Having a language just decide that it would like to change the types of the values flowing through a system is wild. It's one of the reasons that JavaScript is a trash fire.