Its simple the usb3 cables dont have the quality to negotiate pcie 3.0, think about it usb3 cables are made for 8b/10b same as what pcie 2.0 run, but pcie 3.0 use 128b/130b.

Now there is faster standards like USB 3.1 Gen2 or USB 3.2 that do use 128b/130b but i would imagine that they also have much shorter cables, and its not cheap

EDIT: Already at pcie 2.0 these usb risers suck, if you have problems with gpu fallen of the bus errors in your syslog, and your gpu's are not too much clocked, you can usualy fix it by forcing pcie 1.0, so the risers are iffy even for 2.0, you say they can 3.0 I say NO !