This is an infinitely tangential question and obviously will change depending on worldview or religion.
Why wouldnt one live two earthly lives? Arguing against that position is easy; it is the nature of everything we see around us. A fire is no longer a fire once it is doused. A duck becomes shit after I eat it.
Three possibilities:
1. My shit could magically congeal into a new duck after it goes down the toilet out of view (similar to the way Toy Story toys only activate once out of range of humans) - A big logical leap, highly unlikely.
2. The duck, having lost its life, could return into a life essence 'river' or 'source' from which it eventually emerges a new duck in a new body, place, or 'soul' - Also a big logical leap and highly unlikely given our knowledge of the physical world.
3. The duck, once dead, dies, and does not become a new duck. - Makes the most rational sense, repeatable and plausible.
1 and 2 involve presuppositions far out of the scope of the observable world. Using Ockham's razor, three makes the most sense.
If we live two physical lives, why have no memory of the previous life?
Everyone I know with a recollection of a 'past life' was something grand - "I was an Egyptian priestess. I was a Viking warrior. I was a revolutionary."
No one ever says "I was a janitor in a past life." "In a past life, I stuttered and had no friends." "I was an overweight, bossy wife to a frequently absent fisherman in a past life". This lends me to believe that people who believe in them are mostly losers opining over a magical, nonexistent past, similar to the way aged or unattractive women read ridiculous romance novels with fabricated men and impossible situations.
People who believe in heaven and hell don't believe in a recurring earthly existence simply because the life cycle spoke of by the major Abrahamic religions is sequential: birth->death->afterlife.
Why don't these people believe in reincarnation or a possibility of multiple earthly existences: Simple, they are not Hindu or they do not come from a religion that believes in reincarnation.
Whats like going to the UK and asking why most people don't root for the Golden State Warriors - they are from the UK.
916 your posts often try to put a sort of logical or structural bent on religion - the problem is that religion is not based on logic or an overarching structure. Religions evolve largely based on their region and the culture surrounding it, as well as the events of the world at the time, the character and flavor of those to whom it was revealed, etc.
Religions are not like math, and once one makes a logical statement or presupposition based on an observation of one belief or worldview, one cannot simply apply it to other religions.
In other words - why don't people who believe in Heaven and Hell believe in multiple lives - because their religious texts do not say so. I assume you come from a religion that does, so there you have it.
One question I didn't answer is if the mechanism was in place to live one earthly existence, why not two or more? The answer, once again, is simplicity. We see living things on the earth, plant, animal, or human, as having a definite beginning and an end. Simply because a new child is born, a new plant grows, or a new squirrel is born does not imply that the squirrel, the plant, or the person was previously alive.