@Deicide:
So what? 9/10 of the others didn't start willing or expecting to join the crew. It's throughout their character arcs that either they or Luffy are convinced that they want to be in the crew.
9/10 of the others also joined in the same arc as they debuted, and none just stood there awkardly in a corner for 5 years. Also I can't think of a single example of someone asking to join the crew and being refused, so that doesn't really answer the issue, does it.
First, from it being one of the first things stated, because character arcs are meant to take a character from Point A to B. Depending on how the story goes, their beliefs in Point A may not be the same in Point B (like almost every crewmate not wanting to be a pirate, but ending up as one anyway). It's not unconceivable that the journey this time is finding there are more important things he care about than seeing the world.
There's a big difference between a charachter arc and a complete 180.
It's not unconceivable, sure, but woud you really like Yamato willingly extend his lifetime imprisonment and lose the once in a lifetime opportunity to relive his hero's dream in order to babysit a 28 yo ereditary prince?
Third, for Luffy's reaction. He's annoyed by Yamato asking. There's something wrong with Yamato's request. Luffy sends Yamato to Momo instead, twice, and, later, Yamato is not even in Luffy's thoughts when he's gathering strength for the final stretch of his battle. Even Pedro is there, but not Yamato. Oda is keeping a barrier between Luffy and "Yamao" even after 70 chapters.
Can't be serious about this. There was an half page big nakama-grin from Luffy when he came back on the roof and Yamato show that "no need for words i get ya" moment. Many others already pointed out the similarities between Luffy ordering Yams to take care of Momo and ordering Franky to save robin in EL, so there's that too.
And finally, because the story being told is not about Yamato making steps towards getting acceptance among the crew. Or doing anything that shows his desire to be a pirate. He "wanting to be Oden" is never portrayed as something bad, but as a quality. His big scenes and declarations are about loyalty to Momo and Wano. His DF is framed as a guardian of Wano. His flashback is about carrying the will of Oden and the defeated daimyo. He bonds with Momo rather than Luffy and the crew.
He wants to be free, Luffy's journey is all about freedom. His superpower is now explicitly stated to be freedom. Luffy is free and frees people, Yamato was a prisoner and is been freed, i think it's pretty on the nose.
Edit: Also, thinking about it, Vivi is the ultimate example. She stayed with the Straw Hats for multiple arcs, appeared in coverspreads and coverpages, got in the "Versus" Volume cover as a Straw Hat. She was undoubtedly considered "nakama", she was offered to join, she wanted to join, Oda kept the ball rolling until the very last chapter, when finally there was the goodbye. Why is it so unthinkable for Yamato?
Yamato is not Vivi. Momo is Vivi, simple as that.