The reason for this (I should have probably added a note to the site in hindsight), is that WordNet doesn't include definitions for these words in its corpus. This is why the count is less than 10,000: anything that WordNet doesn't have a definition for isn't included. I left a nod to this in the asterisk, but I realise now I didn't explain it anywhere.
> WordNet only contains "open-class words": nouns, verbs, adjectives, and adverbs. Thus, excluded words include determiners, prepositions, pronouns, conjunctions, and particles.
I suppose I could have included them as source nodes (only outgoing), but I think they would have ended up connecting to a whole bunch of definitions, while not providing much in the way of interest.