Because he knew he was walking into a trap.
But his shame over his part in Davros' beginnings wouldn't allow him to refuse. His bravado to their eyestalks notwithstanding, the Doctor is still intimidated by the Daleks. He knows that Davros is looking for him because of the flash we see of him on Karn. Unless other information comes to light, my personal understanding of the timeline is that he had been following the snake-man, heard the summons while on Karn, and then sent the confession disc off and threw his party. I don't think he expected Missy to snag Clara and hunt him down. As Clara said, he wasn't assuming he'd win.
It's the same behavior we saw from Ten between Waters of Mars and The End of Time, and from Eleven between Closing Time and The Wedding of River Song: the Doctor is given reason to believe that his time is up, but he delays it as long as possible in order to have some fun, until something happens that forces him to understand that he can't put it off any longer.