Because he clearly isn’t. In the manga, while Goku is stronger than Fat Buu he is weaker than Super Buu

Even against Buucolo, Goku says Gohan can beat him, Gohan not himself

Buutenks says that he’s the mightiest Majin

and Super Buu remembers Kid Buu

Buuhan then states himself to be even stronger

This is backed up by the Dragon Ball Forever Guide

Translation: Vegito will not let the mightiest Majin lay a finger on him.

But isn’t Kid Buu stronger than Super Buu?

Actually no, he was just undergoing a transformation into Kid Buu and when he does complete that transformation

and before you say Vegeta’s just being arrogant, no he isn’t and even if he is, Goku also says they can take him and he isn’t someone to underestimate someone based on their size.

Goku also refuses to take the Potara Earrings as he was confident that he could beat Kid Buu on his own

He had no problems using them against Super Buu and Buuhan.

El Manga Legendario confirms this

So Kid Buu is more violent but not more powerful.

So no, Kid Buu is not the strongest form of Majin Buu in neither the manga nor the anime continuity. All those statements in the anime about him being more powerful were

A) Filler

B) Only to hype him up, they are not meant to take him literally.

So that’s why people say Kid Buu is not the strongest form of Majin Buu

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