@SimplyBuilds The main menu now features a real background image. If it were stretched to cover the entire screen, it would become distorted and lose its quality. To avoid this, black bars have been added around the image to define its boundaries. On larger monitors, such as ultrawide displays, these black bars are even more noticeable.
You mentioned that your screen resolution is 1920x1080 (Full HD), correct? I noticed you're playing in windowed mode. If you switch to fullscreen mode, the black bars will disappear. However, even in fullscreen mode, they would remain if the screen resolution exceeds the image's aspect ratio, such as on ultrawide monitors.
It's also worth mentioning that The Sims 4 loading screens are set to change soon, showcasing in-game images instead. As a result, black bars will appear on the loading screens as well. Unfortunately, there’s no way to avoid this.