> but why does a code editor need to render on the GPU like a video game?
It isn't just text editors—nowadays, everything renders on your GPU, even your desktop and terminal (unless you're on a tty). For example, at the bottom of Chromium, Electron, and Avalonia's graphics stack is Skia, which is a cross-platform GPU-accelerated windowing and 2D graphics library.
GPU compositing is what allows transparency, glass effects, shadowing, and it makes actually writing these programs much easier, as everything is the same interface and uses the same rendering pipeline as everything else.
A window in front of another, or a window partially outside the display? No big deal, just set the 3D coordinates, width, and height correctly for each window, and the GPU will do hidden-surface removal and viewing frustum clipping automatically and for free, no need for any sorting. Want a 'preview' of the live contents of each window in a task bar or during Alt-Tab, like on Windows 7? No problem, render each window to a texture and sample it in the taskbar panels' smaller viewports. Want to scale or otherwise squeeze/manipulate the contents of each window during minimise/maximise, like macOS does? Easy, write a shader.
This was a big deal in the early 2000s when GPUs finally had enough raw compute to always run everything, and basically every single OS and compositor switched to GPU rendering roughly in the same timeline—Quartz Extreme on Mac OS X, DWM.exe on Windows, and Linux's variety of compositors, including KWin, Compiz, and more.
There's a reason OSs from that time frame had so many glassy, funky effects—this was primarily to show off just how advanced their GPU-powered compositors were, and this was also a big reason why Windows Vista fell so hard on its face—its compositor was especially hard on the scrawny integrated GPUs of the time, enough that two themes—Aero Basic, and Aero Glass—had to be released for different GPUs.