Well, there are a lot explanations. I won't enter into details very much because the thread will become gruesome, but you can do a lot of things with a single plot. Cremation until a few years ago wasn't usual and it was thought in the 19th and 20th that it was important to have a big plot with a deep vault and an impressive monument.
Thanks for the explanation ! 7 billion sounded really strange to me... but I agree that 7 million already is a lot, and it must be very complicated to manage all that.
I wonder if it would be of any interest to try to compile (for example on a web site) a sort of "guide of ballet-related graves", for example with photographs of the graves of famous dancers or choreographers ? I vaguely remember that several years ago, the magazine "Danser" had done an article about some graves, but of course only a few people were listed... Compiling a list probably would be quite time-consuming, especially with all the legal restrictions you mentioned, but perhaps it could lead to interesting discoveries.
One can find surprising things in cemeteries, for example in the small (and otherwise uninteresting) cemetery of my parents' village, there is the grave of Liane de Pougy (1869-1950), who briefly was a dancer (but I think, a cabaret dancer, not a ballet dancer) and a famous "demi-mondaine" of the "Belle Epoque"... but she's not buried with the name "Liane de Pougy" (a pseudonym), not as "Anne-Marie Chassaigne" (her birth name) or "Princesse Ghika" (her married name) but under a religious name (something like "Anne-Marie-Madeleine de la Pénitence") because in the last years of her life she was in some religious order...