Because you need to buy the original product first which costs $100. Obviously if you already have one, it would be just as effective and not be any cheaper to modify your existing as shown in Ken's link.
These were/are standard issue on a Sea Ray. But, OK, I see where you are coming from.
Holding tanks... top or bottom:
I can't speak for the Sport Yacht and Yacht segments, but Vince's post shows how the holding tanks are on all the Sport Cruisers. At least as far as I know for that last 10 to 15 years. Could be longer, but I can't comment on that.
PS: Thanks, Vince! I'm awful at getting pictures off the web and into a post!