Mostly because user interface is meant to expose the common tasks to the user vs API which is supposed to give a low level control. curl is a good example, as a

Mostly because user interface is meant to expose the common tasks to the user vs API which is supposed to give a low level control. curl is a good example, as a tool is fairly accessible, but as the library it gives control on all the aspects of connection, requiring much more knowledge/skill from the programmer. Not every user has this kind of knowledge, but still may want to use curl to do quick HTTP requests. User != Programmer Also, "bash calls" are cool - shell is a great tool to compose many diverse tools written in different programming languages. If all the tools were just configurable via writing code in their implementation language: 1) you would have to know all the programming languages 2) you would have to somehow input the code, parameters are just shorter than code