PS/2 interfaces are actually still quite common for "enthusiast" motherboards, because of the low latency (the simplicity compared to USB certainly has a lot to do with it.) Laptops also tend to have PS/2 keyboards and a "mouse" (the trackpad, which can be switched to a proprietary protocol later for more features) because the EC they use provides that "for free" and that industry tends to have a "it works, why change it and break things" attitude.[1]
I think there was a Ben Eater video debunking the "PS/2 is faster than USB" myth.
That used to be true, but the PS/2 is so much slower (lower frequency) that by the time a package containing the mouse or keyboard info is finished transmitting the usb port started and finished a poll
USB is faster and it is better when there is nothing saturating the USB with traffic and CPU can keep up.
Most of the time there will be no noticeable difference using the USB mouse or PS/2 mouse.
But if CPU is busy and USB bus is saturated, USB mouse will start to be less responsive than PS/2 mouse.
In the extreme cases even USB keyboard will become unresponsive where the old PS/2 keyboard still works fine.
This can make a difference between being able to terminate a misbehaving process and resume working or having to power-cycle the computer.
I frequently have this problem and I'm always aghast that no OS that I'm aware of has a good method to restrict resource usage to ensure the user can still perform essential tasks like terminate a problematic process. Instead, the machine just grinds to a halt and all you can do is sit and wait or else train your finger on the power switch.
https://news.ycombinator.com/item?id=17692499
That said, BIOSes can also emulate PS/2 with USB (no doubt via SMM). This is usually a setting called "legacy keyboard/mouse support".
[1] The notable exception being Apple. Think Different. https://news.ycombinator.com/item?id=12924051