Because we know how to build bridges so that they don't collapse. The laws of physics that govern bridge-building are well known. The equivalent for AI systems? Not really.
We don't know if there's even any danger. All statements so far of any danger are somewhere between science-fiction stories and anthropomorphizing AI as some kind of god. The equivalent of "if the bridge breaks down, someone can be hurt", namely a real, quantifiable danger is sorely lacking here.
Best practices will say things like "you should test it". While we are ignorant, there are just many reasonable things to do. Human biology is not completely understood, but that does not mean medical checklists are useless.
Test it how? What makes it fail? The ability to tell people how to make a bomb? Being able to say what (few) good things Hitler accomplished for Germany? Giving medical advice? Where’s the line?
One thing law explicitly says is full shutdown capability. So it should be tested whether it can autonomously hack computers on the internet and propagate itself. In fact Anthropic tested this. See https://metr.org/ for more.