Unfortunately, a must for Canon UW housing

I have the diving weights now almost a year to the housing WP-DC38 in use and have to say that it is much better handling of the camera underwater with the weights. However, I must differ from the recommended number of weights, depending on the dive site (freshwater / Red meeer / Mediterranean). The weights are dropped me under water again, so from me to point to regularly check the screw.
The build quality is good, so far I had no corrosion. The weights are made of 4 metal discs of 60g to the wheels to keep 2 different length mounting screws and 2 plugs together.
What bothers me about the weights, is that I have to buy this separately. Why not simply Canon sets the appropriate number of weights underwater housings for?