Comment by stevage
I find it very painful that modern computers still have such terrible midi chips, and modern operating systems don't come with decent midi synthesisers. It's a real hassle trying to get reasonable sounding MIDI on either windows or macOS.
Modern computers do not have integrated sound synthesis hardware whatsoever, there has been no point in shipping that since the start of the 21st century. Unfortunately, that does mean that for playing legacy General MIDI content, users are stuck with poor quality software implementations like Microsoft GS Wavetable Synth and the (modern successor of the) QuickTime synthesizer, which were designed for slow computers with little memory in the late 1990's, and were trying not to compete with the high-end hardware of the time (because both license samples from Roland, who made such hardware).
General MIDI as a format basically died for serious computer use in the early 2000's. It's still a feature of some home keyboards and things, but it is a case where they genuinely do not “make them like they used to”. If you want the greatest possible GM support, you have to buy old hardware from e.g. the Roland Sound Canvas or Yamaha MU series, or obtain old software (no longer sold) like Roland Sound Canvas VA or Yamaha S-YXG50.