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.
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)
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:
On the other hand, MacOS9's UI style has aged remarkably well:
https://guidebookgallery.org/screenshots/macos90
(minus the media player, ugh)