When they break up with you because they “want to get their life together “

Why do people break up because they want to “get their life together”, why can’t you just grow and build with your partner? Isn’t that what relationships are for ? Then after the break up, they are still in the same place they were and not even make attempts to “get their life together” lol. They use this time to sleep around and go down the rabbit hole and then few months or years later, they regret their decision and come back begging.