For some times I thought exactly the same about this "missing" functionality…
Now that I develop dsp algorithms by myself I agree 100% with nickrose, it's a tough tough tough stuff!
Even the "easy" basic ones (delay, flanger, phaser,…) are a nightmare within the limited resources of a pedal. Doing the same, for instance, as a VST plugin on a plenty full of power processor is more than an order of magnitude easier.
have fun
