Right but... let's take mountains for a second.
Who lives on Mountains? Well, we have Humans, Goliaths, Aarakocra, Kobolds, Goblins, Orcs, Hobgoblins, Derro, Bugbears, Dwarves, Duergar, Drow, various giants, various dragons, firenewts potentially, Harpies, Meazels, Kruthik, Ogres, Peryton, Manticore, Ettin, Yeti, Trolls, Hags, Balhannoth, and Ki-Rin.
And that is just Mountains, and a taste of the things in mountains that are sentient with a language, and not explicitly from another plane of existence.
Yes, the planet is big, but Mountains are a small percentage of that planet, and we have potentially 30 different sentient cultures on any given mountain. Even if you only take a THIRD of them for any particular mountain range... that is still so many.
And then do this again for the forests, and then again for the hills, and then again for the deserts.... it just adds up.