If you believe in karma...why? And if you believe we're here to learn lessons... what is the point?

What makes learning 'lessons' (which we may or may not retain) realistic or even necessary? Wouldn't it make MORE sense to say that we live and die and maybe, possibly live and die again? What if people who have a hard life (or a great life) have such, because that is what life IS... unpredictable and impulsive?