That depends - it's entirely possible to design and build vehicles using black box engines. What you might mean is "designing an optimal vechicle"? But for most people, they're basically working on simple CRUD stuff which doesn't need to be supra-optimal etc.
(I could make all my internal code supra-optimal, write all my own data structure code, handcraft some assembly, etc. but it would be beyond pointless since 99% of the time it takes to do stuff is in the database server.)
If you didn't understand how an engine worked, you could make drastic mistakes designing the car (and not even talking about making it optimal). Imagine designing a car, but having no idea if the engine is electric or combustion. How are you going to make the rest of the car fit the nuances of the engine?!
You don't need to write the libraries yourself, but you have to know what the library does, and know the internals, and the intended uses of the library. Otherwise, you end up writing your app with an ORM and don't even understand SQL or how a database works, and disaster surely follows.
would you put it in an interview for uber or to get a driving license? no
plus you forget that stuff really quickly if you don't use it. I can barely remember half of those from Uni.