Through almost all of human history, and still in most of the world, it seems obvious that when the body dies - something survives - the 'soul'.
Beyond that basic insight, there is massive disagreement concerning what specifically happens to the post-death soul - Heaven, Paradise, Reincarnation of many kinds, Nirvana, the Underworld... but that basic belief in soul survival is all-but universal.
Except among Modern people in The West. We do not believe there is a soul; and we do believe that death is the absolute and final extinction of a human individual - indeed we believe that a person may be extinguished during life, when their brain is sufficiently or specifically damaged.
Why is this? The answer is clear: modern people cannot believe in life-after-death because they do not really believe in life. They/ we don't believe, they/ we are not really confident and sure, that anything at all is ever really alive.
Not even (or especially not) our-selves.
Modern people are very unsure whether there is any real difference between the living and the dead - or, if there is a difference, then they cannot define it. They know that 'science', technology, mathematics etc. regards everything the same way, and everything as dead.
If 'life' is about chemistry, as everybody in the mainstream of modern culture seems to think; then it is not really alive! After all, chemistry applies to everything equally - with no distinction between alive and dead. Nothing chemical and specifci happens at death, and yet biology cannot define the moment of death, the dividing line between life and death.
(Neither can biology define exactly when a specific new life arises during development; neither can biology define exactly when life arises during the presumed history of evolution.)
Even New Age spirituality agrees - because for them life is about energies, and frequencies of vibrations - which means physics, which means dead things...
In sum, because we treat everything the same, whether it is living or dead - and because that 'same' is to treat everything as dead... then modern people cannot believe in life.
(Of course, they/ we cannot either believe in consciousness or free will - it seems pretty generally agreed that these make no scientific sense, and therefore consciousness and free will don't exist, are not real.)
Since we moderns find that in practice we cannot draw an objective line between living and dead; we fall into the obvious nonsense of regarding everything as dead (including the person making that staement).
The only solution is to accept what we all came-into this world believing - that everything is alive, but in various ways and to various degrees.
And when we re-recognise that everything is alive, we will recover the obvious, commonsense, once-universal knowledge that biological death is a transition of the soul.
Death is not an extinction of the soul, not the end of the soul; but the trans-form-ation of the soul - from one form into another.
Then we can return to our proper concern and conversation - which is to discover what really does happen to the soul at death... What are the possibilities, what are the options?