The problem is closely related to computational typesetting, e.g. TeX, which is an extremely complicated piece of software. While it might be true that music notation is ultimately "simpler" than document typesetting, I wouldn't stake money on that claim.

A somewhat ignorant TL;DR:

- It's probably not too difficult to render simple music in a bland, ugly way, in the same way that it's not too difficult to render ASCII text documents with a monospace font.

- When notation gets complicated it is tedious but somewhat routine to render music in a way that looks like total garbage: things are awkwardly smooshed together or spaced way too far apart; some symbols look "too small" alongside others when using uniform scaling; long lines that have to travel over multiple notes end up clashing and making the score unreadable; etc etc etc.

- Actually getting things right in a general-purpose musical notation program is hard hard work. I don't have any direct experience here but I am sure it's a nightmare and takes constant testing to figure all this stuff out.

I play jazz and don't fully believe in Western music notation :) So I can't speak too directly here on the intricacy. But I would highly recommend reading Knuth's TeX Book (and skimming the WEB source of TeX itself) just to get a sense of how many different decisions a typesetting / music notation program has to make.