Because thats literally how the plot is progressing, Shanks not wanting to meet Luffy until the endgame lol. There is literally nothing supporting Blackbeard and Shanks "need" to encounter each other before this either.
You (and many other people no offense) are being massively hardheaded about this. This can easily just happen all at once on one arc, but you all give yourself major constraints on how expect characters to encounter each other.
The audacity to tell me thats not where the story is going and yet Oda has been beating this in our brains for 2 years already:
But sure sure, Shanks vs Blackbeard is the only EVER highlighted on these pages. Nothing about Shanks wanting One Piece. Nothing about Shanks wanting to meet Luffy. Nothing about Shanks wanting attack Barto. Nothing about the Marines (i.e. Ruokugyuu) wanting to fight RHP. Nothing about Buggy wanting to sneak attack OP behind the other Yonko's back.
Sure buddy, SURE.