There are 16 advertising dealers here. They can list whatever they want. Some list a lot, like B2C and Adiamore, some list rather little, like Leon Mege. It’s their choice. There's no requirement that they list all that's available to them and most don't. They pay a fair amount to do this and those who choose not to pay get nothing. That’s the way advertising works. There are tens of thousands of diamond dealers in the world and only 16 of them advertise here.
I didn’t look at every shape but there’s just over 280.000 round stones offered. There’s a fair amount of duplication in there where the same stone is being offered by several people but I think it’s a fair guess that there are over 100.000 unique round stones for sale and probably 40,000 in other shapes.