A lot of the ease of using Discord comes from the fact that every server just works, and you can join using a link. If people could host their own servers I'd imagine you'd end up with poorly-hosted servers that would degrade the overall experience.
And highly increase the personal privacy and use cases. Let's be real this was a monetary decision and nothing else.
Theres only so much even an inexperienced admin can do wrong when hosting something as basic as a chat platform. The various limits a potential machine could have are very obvious immediately. If people could host their own servers the artificial limitations discord is using to extract money from its userbase would be even more incomprehensible, and the network effect wouldn't be nearly as strong because every instance would presumably have its own account registry, both points will prevent discord indefinetly from ever allowing self managed instances.