it's such great a rejection of all the stale, boring, "clean" UI convention that we're drowning in today.
it's not just nostalgia - it's fun, it's rebellious, it has real character. it shouts "I'm having fun, why shouldn't you?"
I don’t think more modern/clean designs have to be devoid of fun/rebelliousness or lack character (I’m working on a site that IMO intersects both, it’s very much a WIP but feel free to check out [my handle].github.io if you want to see it in its infancy). But it certainly does tend to hew toward the easy.
The only thing I disagree with in your comment is that it necessarily has character. At a certain point the retro design trend will be so commonplace that it’s also hewing easy and a fairly straight shot from “I want brand to be fun”. I hope it doesn’t reach that point though, because I quite like being surprised by the occasional 90s-style graphics.