USB ports not only existed, but were standard in new PCs.
A quick search shows peripherals and monitors with USB hubs in PC World May-1998 (4/5 months before iMac launch). From that months number (Pages 172-184):
"UNTIL RECENTLY , USB hardware was as scarce as hen's teeth . Now, after almost two years of foot-dragging, hardware companies are fina ll y delivering peripherals. We tested 18 of these new products"
Among the products there are keyboards, mouses, joysticks, cameras, scanners, speakers... you name it.
>Apple wasn't the first to market with USB (and Snell didn't claim it was), but Apple most certainly drove its mass-market adoption.
Sure they were scarcely used, support began with Windows 95 OSR 2 in Aug-97. And relatively new computers were only a fraction of the market. But the real market driver was the PC.
https://vintageapple.org/pcworld/
Edited to add link and page reference.