Structural engineering is much like calling library functions with certain parameters and then making sure it all works and is safe. You know how what you are constructing differs from existing buildings and use that to decide what to do. You don't (usually) invent new ways of building things or building things that are unlike anything that was built before.Therefore such projects are well-known engineering practice, and your skill and knowledge of those practices can be tested and you can be licensed.

Some software development is like that too but most is not, you are creating new designs all the time because your software is for new purposes, not to replace something existing with perhaps a slightly improved version. It's hard to license anybody in "inventing new things".