The older you get the more difficult it is to conceive children as well as raise them. Given that most people have a partner that is roughly the same age as them, and that there would be a much smaller percentage chance of pairing with someone with the same long life trait, the period of time that they are likely to have children probably stays consistent with societal norms. Therefore living a long life wouldn't increase the number of offspring and wouldn't provide any evnolutionary benefit.