system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we still use USB and not fibre optic for file transfer. My Soundbar uses fibre optic, so it can't be a cost issue.

It takes ages to copy from one hard drive to another using USB, surely fibre optic would be faster?
gpt
USB is

* cheaper (both cables and controllers)

* bidirectional by default (SP/DIF is unidirectional because it is only 1 fibre cable)

* can carry multiple signals at the same time (again, your SP/DIF only carries one. Alternative is multiple fibers, or multiple colours per fibre, but that second one increases cost exponentially)

* can be split into multiple USB ports through hubs (fibre can only do point to point links)

* has more compact controllers and interfaces (optical requires very deep jacks to ensure correct optical transfer between cable and socket)

And last but not least

* can carry power, because it's made of copper wiring

which is VERY important for just about any peripheral. Without USB powering a peripheral, you'd have an extra power cable. In fact, they are drastically increasing the power transfer capability in USB Type C cables, so that soon you'll be able to charge your laptop and tablet PC just like you do your phone.

as mentioned in this thread, fibre is only used where the limits of copper are a hindrance. This is

* Where speed is very important

* Where great distances need to be bridged (think kilometers, without amplifiers)