system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why personal computers mostly use graphical user interfaces and mainframe/servers & supercomputers are mostly controlled through command/text line?

I know why PCs use GUI that is interactive for personal usage but for the sake of making a complete el5 question for others, I've included it.
gpt
People who use the latter category are mainly IT professionals and computer scientists who a) may not need the more user-friendly interface, which saves development costs, and b) might actually be restricted by a GUI. Pure command line interfaces give the user complete control of whatever the program in question is capable of, generally speaking.

E: the resource cost of running a GUI may also matter, as another comment says, especially when you're in the business of optimizing algorithms.