The spec isn't proprietary, you can download all 482 pages of it right here[1]. It's just that a USB controller needs firmware and none of the vendors making USB3 controller chips have open sourced their firmware. Most chip vendors are not exactly huge fans of open source and likely consider portions of the firmware to be trade secrets.

[1] https://www.usb3.com/whitepapers/USB%203%200%20(11132008)-fi...

No, it's just complicated - easier to implement it on a reasonably general purpose core with firmware than to bet on getting it right first time in dedicated hardware.