Are you sure that your machine code gets run properly? Do you test the CPU hardware and all the logical gates and whatnot so that they respond to the schemes and it passes the electric current properly?

Like xkcd 378, we could follow this logic quite deep. At some point you just have to trust, or at least assume that something is going to work, without properly verifying it yourself.

Smarter people than me have essentially dedicated their life to some extremely specific area of computing. It's not reasonable to assume that I could and should deep-dive into every possible nook and cranny just in order to build an application. I already have to do a whole lot of assumptions because of my hardware used for development, my OS and software tools like my IDE, task runners and yes, external libraries.