Because no single language can (nor should) be a good match for solving all types problems. For this reason it's better to be fluent in 5 (or so) small languages than one big language IMHO.
Most used first: C (C99 is different enough from the common C/C++ subset that it counts as its own language IMHO), Python, C++ (up to around C++11), Javascript, and recently more and more Zig. Less then I would like: Go (since currently I don't do much server backend stuff).
PS: forgot Objective-C, for coding against macOS APIs.
>"Less then I would like: Go (since currently I don't do much server backend stuff)."
I do loads of server backend in C++. Never felt that I need anything else for this kind of stuff. Sometimes due to client's insistence I did it in other languages but it was their choice.
