Why have wildlife experts only estimated the number of endangered species in the world?

The problem starts already earlier:wildlife experts can only estimate the number of  species in the world generally!
 This is the case because there are simply too many species, and it takes time to documents them; because some areas (ocean bottom for example) are very hard to access and because sometimes it's hard to know whether we're talking about a new species or a variation within the same species.

so if we even don't know how many species there are, it's even harder to know how many are endangered.