Board Thread:Questions and Answers/@comment-30771283-20171021140407/@comment-29820886-20171022061915

Also, they had pass 5 years into the wall, for peoples who have 13 years to live it's a lot. RBA probably doesn't know that Eren was the founding but they go anyway because they needed to bring back something to Marley, be it the attack or the founding for justify the last five years.

They had 1/2 probability of Eren been the founding, but they desperatly wanted to go home and bring something back with them for made up with the loss of the jaws (Annie in particular, wich is probably the reason why she had go alone), if they were sure Eren was the founding they would had go together since he is their objective.