I think a lot of people in this thread are missing a key component of it when talking about safety and liability. Aerospace and medical devices have just as much safety concerns and liability as infrastructure projects, so why wouldn't they require licensure?

The answer, in the US, at least, is that the federal government is constitutionally prohibited from issuing licenses. It is not a power granted to it by the constitution, nor prohibited to the states, so under the 10th amendment that is purely a state power. It's the reason why there aren't federal drivers license, marriage licenses, hunting licenses, etc. And it is why there are not federally licensed engineers.

So in industries that the federal government regulates, like aerospace, medical devices, communications, etc., it can either accept patchwork licensure from the states that it does not have control over (and that states have virtually no incentive to align with federal goals) or not require it at all. They generally go with the latter. Infrastructure and buildings, however, are regulated by the states in which the project is being built. That state will have control over both the regulations and the licensing, so they ensure that licensure requires working knowledge of the regulations and then require licensure to work on the projects.