Because no one refuses Mama's invitation.Almost zero. Yonkou are by definition enemies of each other and having two meet, regardless of the situation, is enough reason to cause the entire world to be on high alert as shown when WB and Shanks met. There's also no reason for Shanks to be there in the first place.
nah , the fat ass ***** would be pissed bc she wants luffy in her crew xDI actually think someone of his caliber will show up. Because luffy v big mom would be bad for everyone mostly luffy though so if shanks came and claimed that luffy was his subordinate it would be a solution to that problem