I think you will find that all the major tool makers will sell their tools unhandled. Shops tend not to stock them because they don't look so tempting as their handled cousins. In my opinion most commercially available tools...and every roughing gouge I have ever come across ...are underhandled.
As for the reason for longer handles giving better control...
When cutting the tool rest acts as a fulcrum and the tool as a lever. The length of that lever beyond the fulcrum works against you while that on your side works in your favour. Therefore the closer the toolrest to the work and the maximum length of handle gives you more leverage and hence control.
Brian