The main difference is that your monitor is stationary so fonts are rendered onto a grid of physical pixels in a fixed position. A monitor in VR is a flat plane

The main difference is that your monitor is stationary so fonts are rendered onto a grid of physical pixels in a fixed position. A monitor in VR is a flat plane that is not aligned with the physical pixels of your headset, so it has to be re-rendered constantly and shifts around on the pixel grid causing sparkles with the slightest head movement.