Buyers were looking for one particular moment
A footage buyer rarely wants every clip of a lion. They're after one behavior, in one kind of setting and shot one way, for the sequence they're cutting: "slow-motion footage of a lion stalking prey at dusk".
The marketplace serves filmmakers, broadcasters and global brands, but finding that moment depended on someone having described it first. Contributors annotated every clip by hand against a detailed schema, covering species, behavior, habitat, location and technical attributes, before it could go on sale. So the filmmakers held far more footage than they could ever submit, and the library could only be as big as the annotation allowed.