Your 3D TV or VR headset stretches the image back out, resulting in a loss of horizontal sharpness and a slightly softer image.
In an SBS video, the frames for your left and right eyes are placed next to each other. Your device then splits them to create 3D depth.
If you are watching on a modern VR headset like the Meta Quest 3, always look for Full-SBS or 4K SBS files to match the high pixel density of the displays. For older 3D TVs or mobile-phone-based VR setups, Half-SBS is usually sufficient. Best Platforms for Legal and Free 3D SBS Movies