The main reason for it is that blurays of old content sell badly. Really badly. Initially they were mastered properly but shitty sales convinced companies to go with cheaper processing routes to avoid making too little profit for production. What they should be doing, and does happen on extremely popular old content, is to resample the original analogue master into HD. These look fantastic although are sometimes interlaced, Planetes being a good example. What happens more often though is that the DVD master, which was made 10+ years ago with poor analog-digital conversion techniques, is upscaled (badly) to "HD". This is why the blurays are blurry, blocky, have incorrect colour, and generally look bad. You can get better results by upscaling the DVD yourself. In the end, studios are lazy and on tight budgets, so they won't often go for a full ADC master when they can use some Sony or Qtec crapware to upsample a DVD they already have in digital.
Re coding, I would recommend avisynth in a Windows virtual machine for you. It's possible to run vs in OSX but quite difficult to write a filter. A python based filter would be particular slow (unless using cython, but may as well just use raw C then) and you'd likely get better results using someone else's filters.