Ground loops. Stages are very electromagnetically noisy, mainly because of lighting dimmers. If you connect a mains-powered computer to a mains-powered master keyboard via a USB cable, you've just created a big antenna for all of that RF noise. That noise will travel through the ground plane into your USB audio interface.
In that environment, isolation is a constant concern. Connections that are inherently isolated make fault-finding far more straightforward and obviate the need for external isolation boxes.
Only solved in the sense that you can buy additional hardware for ground loop isolation for USB and audio connections. Ground loops and RF interference are a persistent and recurring problem in pretty much all USB audio setups, except perhaps when you're solely using relatively high-end hardware that has been designed specifically for audio (cables included). Without isolation, all it takes is one bad component to ruin the whole setup.
For even a simple USB audio interface setup connected to a single audio source I would recommend isolation and balanced audio cables.
It's a "solved problem" in the sense that it doesn't matter for the use cases USB was designed for. Most people are using USB to connect a few devices at most and over short distances, so these effects are negledgible. But that is not true for stage audio.
In that environment, isolation is a constant concern. Connections that are inherently isolated make fault-finding far more straightforward and obviate the need for external isolation boxes.