It's true that the analog part is not easy, but from my experience also the digital one can be as complicated. Take for example effect units or samplers, they use a lot of proprietary algorithms running in custom chips which can be very hard to understand from the outside and get right. For example emulating the digital part of the Yamaha FM chips and all their quirks, or the Roland D50 chorus/reverb ICs, or the interpolation algorithms in samplers.
Btw, some MAME components actually try to approximate the analog behavior, like for the CEM filter of the Ensoniq ESQ-1: https://github.com/mamedev/mame/blob/master/src/mame/ensoniq...
Obviously though it's a thing that takes a lot of work to get right though, so it's just a first approximation