The short answer is that it is considered a passing touchdown because it is, by definition, a touchdown that occurred on a forward pass. The receiver would still be credited with a "Receiving Touchdown" as well as the sum of the yards credited to his net total. This is simply the definition of how the respective statistics "Receiving Yards", "Passing Yards", "Receiving Touchdowns", and "Passing Touchdowns" are determined.
There are, however, different advanced statistics that reflect what you appear to be seeking, a seemingly more proper distribution of credit on such a play. A stat like "Air Yards" for example, reflects how far downfield the Quarterback's throw traveled before caught by the receiver. This, along with Yards After the Catch or YAC, can be used to more precisely illustrate how a pass play gained its yardage.
I would caution however, against giving the receiver all of the credit for yards after the catch, as often an accurately thrown ball by the QB is a big part of the reason for a long gain after the completion is made. It can be interesting to look at air yards however, for often a QB can inflate his stats simply by throwing passes that are essentially elongated handoffs to skilled receivers who turn them into big plays. If it works, it is certainly a benefit to the offense, but speaks more highly of the receivers and the scheme than it does of the QB.