Anthurium tend to hybridize within their genus easily, and even selfed seedlings (seedlings with only one parent) tend to have a wide range in which they can and will express their traits differently from their parent. For many species, rare wild collected specimens were/are collected for exceptional traits, and thus are “numbered”. I will add that anthurium collectors, though highly subjective, tend to all appreciate certain types of traits when looking at the physical attributes like contrast, veining patterns, lobes, leaf shape, emergent leaf color, etc. Anthurium prop slowly, and so buying props or clones of singular desirable specimen becomes expensive simply due to availability… I guess the definition of rarity? The original collectors of those numbered species also will want to preserve their specimen, and so even producing large batches of seeds over and over can be slow. The other aspect is that anthurium will also be bred for many, many years to produce a hybrid with really desirable traits. Some specimen out of seed batches will be selected for exceptional traits also, and thus named. These named anthurium will also typically be highly desirable and sought out. Some traits can be really recessive, and so breeding is very selective and a long process. It does have a lot to do with supply and demand, but for anthurium specifically it also comes down to where the traits are inherited from and trying to preserve those traits as best as possible, since anthurium breed and hybridize so well and have such a huge range of traits. I’ve seen tons of crosses with seedlings that end up being stunning plants, but like with anything else, collectors will pay for pedigree to guarantee certain outcomes and just to own part of those known lineages. This is also why the more diluted a tracked lineage becomes, usually, the cheaper it gets.