433MHz RF transmission only working when holding usb cable in the air
Yup, all those dangling wires are part of the overall antenna system.

At 434 MHz, half a wavelength is less than 14 inches, or just a little more than a foot.  Anything approaching that size makes it no longer a lumped system, so the usual simplifications of this wire connects that signal to here no longer apply blindly.

Your RF receiver ultimately sees whatever signal is different between its antenna input and its ground connection.  From the RF point of view, it doesn't really matter whether you think of most of the signal being on the antenna or the ground.  Those long wires to your USB converter are acting as antennas, particularly the "ground" wire.  Note that ground is actually a rather complicated subject when you don't have a lumped system anymore.