It is largely for the 'convenience'... as dealers are there from all over and were not able to do all the checks required of stores. I think that it 'could' be required to have a database made...with internet access... vaguely like credit card validity is checked immediately.
A license # of BOTH buyer & seller could be attached to any sales... and restrictions on purchases could be enforced... just as my grocery store can monitor whether I am going from store to store, buying sale items in excess of stated limits.

They 'could' design such a system... they just don't want to.