Because Master Poof is not Kaminoan. Per the Star Wars Databank, he is Quermian:
A gaunt, long-necked Quermian, Yarael Poof had two brains and four arms, which he kept hidden beneath his Jedi robes. Poof was a member of the Jedi Council during the final years of the Republic, and was in the Jedi Council Chamber when Qui-Gon Jinn returned from Tatooine with the extraordinary news that the ancient Sith had returned to challenge the Jedi once more.
Although they cast a similar silhouette, Quermians and Kaminoans are quite different:
Here's a better picture of Master Yarael Poof:
And here's Lama Su and Taun We, from Attack of the Clones:
As you can see, they both share an elongated neck, which makes them appear very similar in low-visibility situations. However, there are a number of differences (both visual and non-visual):
- Master Poof has a more humanoid head (although he has no nose1), with a humanoid shape and humanoid eyes. The Kaminoans, on the other hand, have more fish-like heads: large black eyes and cranial ridges being two prominent differences
- Skin tone is dramatically different; Master Poof has a pigmentation similar to a (very) pale human, while the Kaminoans are more stark white
- As mentioned in the Databank quote above, Quermians had two brains and four arms. Although we can't confirm anything about Kaminoan internal organs, we can fairly confidently state that they only have two arms; with their predilection for form-fitting garments, where would they keep any extras?
1 He's probably really tired of that joke, too