The main issue with AVI and other film containers is that it is designed to work with many codecs and pixel formats, which it good for general purpose films, but requires astronomy software to handle a large array of actually different file formats. General purpose film software are often not well equipped to handle 16-bit per pixel values or some uncompressed data formats. With SER, only one file format handles it all, that's why Siril for example is now developing processing only for SER.