From SN 12.61:
"It would be better for the uninstructed run-of-the-mill person to
hold to the body composed of the four great elements, rather than the
mind, as the self. Why is that? Because this body composed of the four
great elements is seen standing for a year, two years, three, four,
five, ten, twenty, thirty, forty, fifty, a hundred years or more. But
what's called 'mind,' 'intellect,' or 'consciousness' by day and by
night arises as one thing and ceases as another. Just as a monkey,
swinging through a forest wilderness, grabs a branch. Letting go of
it, it grabs another branch. Letting go of that, it grabs another one.
Letting go of that, it grabs another one. In the same way, what's
called 'mind,' 'intellect,' or 'consciousness' by day and by night
arises as one thing and ceases as another.
The body appears to last for many years, and then becomes old, ill and drops off. If one associates his self with the body, at least, he will see that the body must drop off and will become disillusioned with the body.
But the mind (including intellect and consciousness) changes much more frequently than the body, but has a seamless appearance. It is therefore not unusual, that one considers his same consciousness wanders on from youth to old age in this body and then wanders on to another body in future. This is what the monk Sati mistakenly thought in MN 38.
So, if one cannot see that the mind or intellect or consciousness is not permanent, then he does not have a reason to become disillusioned with it.
No religion preaches the immortality of the physical body, because it obviously perishes. However, many religions preach the eternity of the soul and the continuity of consciousness through the eternal soul in other bodies, which are physical or celestial. They say you can meet and talk to your loved ones in heaven, after you have passed away. The reason for this is that they cannot see how consciousness is impermanent. That's because it's not obvious.
MN 38 has a discussion on the impermanence of consciousness (i.e. eye-consciousness, intellect-consciousness etc.).
Everyone tries to find something permanent to associate their self with, so that they can be permanent. You can find a discussion of this in the River Sutta.