Our mortal life on earth is designed (by God, the creator) for experiencing and learning. That is why all is transient, why nothing lasts.
In mortal life we are meant to have an experience, to learn its lessons; and then we are meant to have other experiences. Nothing repeats exactly. Nothing can be held-onto.
In other words, mortal life is well designed for our benefit - as would be expected from a Creator who is our loving parent.
Heaven, by contrast, is a place for eternal commitments; a place for strong, solid Love: more accurately, the work of Heaven is built-on, built-from, strong and solid Love.
Therefore, Heaven is not a place for learning.
Since Love is the most important thing in God's creation; we could say that mortal life - not Heavenly life - is the place for learning about Love.
God's creation is 'bespoke' - individually tailored to the specific needs of particular persons.
And some people, some spirits, need to learn more about love than others; before Heaven. Perhaps some do not need to learn anything at all - they are incarnated and then die. Others need to learn a lot if they are to have a chance of being capable and willing for eternal Heavenly life; so to give them the best chance, their mortal life is organised accordingly - perhaps it is prolonged, perhaps they experience a lot of certain types of experience with respect to love...
Exactly that which makes it possible for Heaven to be Heavenly, also makes Heaven a poor place for learning.
Conclusion: There is good reason for this mortal life; and for the transient nature of this mortal life!