Because he know how it work and how to master it
There are some things that are part of the body and some that are part of the soul. He takes anything with him that is related to his soul (with two known exceptions up to ch375 of the novel, where I am atm), but leaves anything related to his body. Low level cultivation is not part of the soul, but part of the body, so he leaves that behind, but later there will be cultivation levels that become part of the soul, which he will take with him. There will also be treasures he can take back, which will happen much sooner than the cultivation being brought back.
So when he dies, he just keeps where she stopped in past life? Like he died when getting those six senses thingy and when he returned he still had them