Why didn't the speaker say "the rarest"? Is it okay to say "the rarest"?
Yes, you can say "rarest" in this case. "Rarest" is a superlative of "rare" and would be perfectly fine to use here. However, it is also correct to say "the most rare". "Rarest" and "the most rare" mean exactly the same thing. Most native speakers would probably say "rarest" here instead of "the most rare" because it shortens the sentence and makes it flow better, but both are perfectly fine to use. Ex: Some of the most rare species in the world are found in the tropics. Ex: This is the rarest type of dolphin in the world.