Because both devices have to support USB 3.0 or better and many don't, just because you connect USB 2.0 to 3.0 doesn't mean it will run at 3.0 speeds. Also both devices have to be capable of reaching the max USB speeds.
The rest is you are using a poor software to transfer the data. I have no problem getting max speeds over USB 3.0 with compatible devices and a good copy program.
Many modern devices are still only USB 2.0. You're better off taking that SD card out and using a USB 3.0 reader at which point you would need a very fast Ethernet to match those speeds. Hell even the regular 100Mb/s ethernet majority uses has only 8-10MB/s while USB2.0 can do around 20MB/s. Or god forbid slow WiFi.