Because data which is written to (external) hdd is not cached, but written directly to the hdd, and an usb drive then also has to work the index (fat, allocation table) for every file, so it has to scratch the hdd back and forth from file data to file allocation data. Because if failure when cached, files will be lost, hdd corrupt, etc.
Internal drives can be cached. Then it can like burst the files to hdd.
In fact they do it to slow you down, like the whole commercial computer business.
Further drivers on usb hdd drives are not intelligen, nor is the hdd.
People want cheap stuff.
Don't they say it on the description of the hdd, no, off course not.
They only mention the large file transfer rate.