The natural tendency in almost any software is to keep adding and adding, while rarely throwing anything out. More features, more code, more supported platforms, more supported languages, more this, more that, more, more, more.

If it was a physical product, you couldn't keep making it bigger and more complex ad infinitum, because making a physical thing bigger takes more material, and bounded physical resources would be consumed. With software, it's all just bits, and computers can hold a lot of bits.

This leads to bigger problems than just git running slowly.