A couple of reasons
There was an article a while back recommending a "KISS" tank installation above the waterline, drain fitting on the bottom to dump using gravity, and always going through the tank, whether holding or flushing directly overboard. I'm not thrilled with that arrangement. It may be simple, and it does eliminate the need for a pump...but it also means that waste is always sitting in the "dumpout" line under a certain amount of pressure when the seacock is closed--which is all the time in coastal waters, increasing the potential for odor permation. Going through the tank all the time to go overboard at sea means keeping a below waterline thru-hull open all the time, and the required vented loop adds still another component that has to be maintained and can create problems. And then there's the matter of always being forced to maintain the tank for odor control etc whether you're in waters where you have to use it or not. Everybody is looking for a way to make systems maintenance free, but most of the "improvements" just replace one set of problems with another set. They may appear to eliminate some maintenance, but in reality they only make it easier to ignore needed maintenance.