"lickable" I think was the phrase Steve Jobs used for it.
Remember that Apple had just made a killing selling jelly-bean aesthetic iMacs (running classic Mac OS 9, even). The Aqua stuff matched the physical case design of those machines.
It all goes along with the late 90s / Y2K times, intense colours. "Run Lola Run" and Fifth Element and late 90s club culture and whatnot. Nothing restrained about that era in terms of style. It was boom times end-of-millennium and that aesthetic continued even for a bit after the .com crash and 9/11 deflated the tires.
Also as others have pointed out, it had to do with "we can do this now'; the graphics hardware and the software stack (showing off "Quartz" etc)
Funnily enough I was playing with a couple of old PowerMac G4s a couple of weeks ago (actually wiping them for disposal - though I've since decided to keep at least one!)
I realised very quickly that I'd forgotten how good the OS9 interface is, and how well it stands up against today's offerings. Trying out a lookalike theme on Linux [1] is definitely on my todo list:
Modern macOS is remarkably usable compared to that time, especially with a few tweaks in place (Magnet app for window management, three-finger dragging, etc.)
It's really easy to forget how far we've come. Going back to an old OS and trying to get some real work done can be eye-opening.
I'm still waiting desperately for some DE to get back to this. GNOME 4x has in some ways but obviously not in others... Please someone re-make an OS 9-like UI for me.
Hard to do on such low resolution displays where every single pixel stands out. It would have been better to avoid italics completely in most of the UI.
On the other hand, MacOS9's UI style has aged remarkably well:
https://guidebookgallery.org/screenshots/macos90
(minus the media player, ugh)