Why can some PC games be resized freely while others support only fixed resolutions?

Fullscreen and windowed modes can use different presentation paths, and the available resolutions may depend on the monitor, operating system, graphics API, and display mode. Still, modern engines are generally capable of rendering to a dynamically sized window. Whether players can resize it freely comes down to how the developers handled viewport updates, UI layout, aspect-ratio rules, and minimum or maximum window sizes.

It’s mostly an implementation choice, but arbitrary sizes do add complexity. The 3D rendering itself usually scales naturally: the game can update the viewport and render to the new dimensions. The harder part is the interface and layout. Menus, text, HUD elements, aspect ratios, camera framing, and split-screen views all need to behave sensibly at different widths, heights, and ultrawide formats. Developers may therefore support a minimum and maximum size, or provide only a tested list of resolutions to reduce bugs and testing work.