Good question and one that has been a heated topic on many forums!
In some cases, the seller makes more money!
That is what drives people to do it and then others have hopped on board selling and parting rarer drums which in those cases will probably bring less money.
Splitting up a silver sparkle set might complete 4 other sets, but splitting up a rare finish has less of a demand then silver sparkle since less people are trying to find matching drums.
So it has back fired on some sellers that do not understand what they have.
Now with all that said there are exceptions to every rule and drums that you would never think should sell that high do, because two people have to have it at that moment in time.
There are also guys that take good drums and part them out down to the washers. Another heated topic for another time.
In the end sellers search Ebay and see others parting and splitting drums so they think it is a good idea and do the same thing.
Dealers, and most vintage collectors would never do it.
I hope that helps
David