Except he didn't have the time to meet with them, who where all far from Terra (Except Dorn). Second, most of his time and effort was spent containing the Daemons from entering (Magnus shit happening).
I loved the Thousand Sons as a chapter, but the fact that Magnus was always deceived since the beggining about the nature of his sorcery, well... He messed up ROYALY. If it wasn't for him, the Emperor might have had time to FINISH the webway thing.