HaShem created an Olam HaZeh/Yemos HaMashiach/Le'asid Lavo world, and He also created an Olam Haba/Gan Eden world.
Why is it necessary to have two worlds that do not interact with each other? What is the point of keeping the living from mingling with the dead?
Granted that after the sin Adam introduced death into the world, but perhaps
(1) HaShem could have made it that after 120 man becomes some sort of an ethereal state in which he gets rewarded and punished for his previous actions while still being able to interact with this world.
Or (2) perhaps there could be some settlement in the middle of nowhere where the "dead" live, and they can interact with the outside world.
Or (3) perhaps leave it as is, with two worlds, but allow some level of interaction between them without sorcery.
Why make two worlds that are completely separate from one another?