On Source Engine 1 you would see mountains of ancient code that nobody understood anymore. No one human understood the whole thing. It was impossible. Adding major new graphical features to the engine without breaking the universe became enormously difficult.

While adding new features, you couldn't just wade in and disable or break code you didn't understand or think wasn't actually needed. Odds are that code was used somewhere in a way you couldn't predict. This happened to me on CS:GO in surprising ways.

Also, if you touched the renderer, even in a simple way, and a team later encountered a rendering bug, you would be blamed and have to fix it. Even if the bug had nothing to do with your change. This taught programmers to not change anything unless absolutely necessary.

Adding cascaded shadow maps to CS:GO (on Source 1) was a multi-month *herculean* effort. The stress on that effort was off the charts.