I just dont get why people choose Python and not e.g. Go for high performance problems.

my initial choice was to use Rust for this actually (Probably should've too :P) but i went with python for an initial mvp/skeleton for a future rewrite

Go is pretty good at performance, but pretty bad at expressing domain-specific logics. Python is the opposite, but once you have isolated the parts that need to be optimized, it's quite easy to rewrite them in a native language (in particular, the Rust-Python bindings are really good, although in this project, it's C++).

Go is not high performance enough. Like what others said, you implement the high performance part in C++ and use python to glue them.

Python is a very convenient skeleton for gluing together high performance modules that were written in C or cuda. Writing boilerplate code in those to adapt them to your project is much more inconvenient.