Because it CAN!
The design of most current fittings isn’t very good. Basin wastes come with nothing to stop water running down the grooves of the threaded part.
The two commonest solutions are to use sanitary silicone between the thread and the ceramic of the basin, or to use a Basin Mate. 
This replaces the rubber washer which goes against the bottom surface of the basin.
They cost a couple of pounds and are available at many plumbers’ merchants.
They squash down to about 12mm thick.