Because of his worry about being caught and having to give both of the two plants over to the people who caught him, the senior didn't want Alex to come with him.
The World Tree and the Nine Yang Divine Tree were far too important of a treasure for it to get into the hands of the wrong people.
Because of the lack of Qi in the world, as soon as someone with the wrong intentions got their hands on the World Tree, they could use the tree to get stronger themselves and could restart the war to try and gain the remaining Immortal realms and spirit realms they didn't get their hands on before.
That was the reason why he wanted him and Alex to leave separately.
"I don't know how possible it is for you, but I want you to leave this world as late as you can," the man said. "Let some other people start breaking through to Immortality first so the Immortal world can begin having precedence of Immortals coming out of this realm."
