It's also a basic implementation, doing a divide per sample to renormalize and not taking advantage of the symmetry of the kernel.

Even on a GPU, you wouldn't use an algorithm this simple in production. For small radii, leveraging bilinear filtering halves the number of samples. For larger radii, hierarchical pyramid approximations are popular.

Probably a reflection of the age of that code along with the usefulness of the feature. If no one actually needs the video blurry, then it doesn't matter how efficient the filter is.