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

It is mostly an implementation choice, but arbitrary resizing does add complexity. The renderer can usually adjust the viewport and render at the new size, especially in a 3D engine. The harder part is the user interface: menus, HUD elements, text, and panels all need to remain readable and correctly positioned across different aspect ratios and window sizes. Developers may support a minimum and maximum size, or just provide a short list of tested resolutions to reduce bugs and testing effort.

A larger window can require more pixels to be rendered, so performance may drop as the window grows. That is separate from whether resizing is technically possible, though. Many games can render at any size but deliberately restrict the options for consistency, performance testing, or because the interface was designed around a particular resolution or aspect ratio.

Fixed resolutions can also be important for games with pixel art or other resolution-dependent visuals. Scaling to an unusual size may make pixels blurry or uneven, while a carefully chosen integer scale looks much cleaner. Modern 3D graphics generally scale well, but the UI and visual presentation still need special handling for ultrawide screens, multiple monitors, and unusual window shapes.