the grandmother effect pretty much boils down to „females survive past reproductive age because they help with grandchild rearing“. well here’s a few reasons why framing it only like that is lopsided:

while primates do in general not show reproductive senescence long before death (baboons being an exception with mild tendency to become infertile before death), there are mammalian species who show the same pattern as humans, for example killer whales.

killer whales show reproductive senescence well before death, yet their calves only stay with the mothers for 2 years. so there is likely no significant grandmother effect, as the young become independent very fast.

so we have established that humans are distinct from most primates in reproductive senescence, yet we are not taking into account that in all primates, females live a bit longer than males.

it’s however not a massive difference in age at death, notable however is, that primates with the least aggression in males (murikis) show the smallest gap between female and male life expectancy.

life expectancy in primates seems also correlated with age at first birth for females, with gorillas and chimpansees being the closest to humans regarding that, chimpansees often past 10 years of age.

the question here seems not „why do females live past reproductive age“, but „why does reproduction cease in human females early“. in the K!ung tribe, females past reproductive age are 42.5% of adult female population. a significant number indeed.

looking at other primates we see that while other forms of organization are also common, the one male system seems quite pervasive. one adult male with a group of females, sometimes one or several helper males of low hierarchy, other males organize in separate bachelor groups.

so there seems to be a certain tendency to favor harems, meaning more females than males in a social group, as it reduces male aggression and infanticide. bonobos in contrast keep the peace in mixed groups with sexual promiscuity.

so in all primates male aggression is a problem that needs to be kept at bay through social order, but also through the presence of enough females. if female monkeys just died early, the males would significantly outnumber them, likely resulting in a rise of aggression.

a sex balance in groups that favors more females over more males means nature had no reason to let females die early. it would also mean that males would have to take over more female roles in the group, which would have prevented humanoids from gendered occupations.

gender roles however made primates very successful, as it was an advanced form of social organization for the species, leading to a distinct sexual dimorphism regarding size, strength etc. dimorphism is highest in terrestrial living polygynous primates.

this dimorphism obviously rendered females more vulnerable, another reason why too many males would be problematic and lead to increased stress, disease and mortality in the outnumbered females.

so, why do females lose fertility?
maybe it was a health benefit at that age to not carry another child while caring for your others still. maybe it‘s reducing offspring. maybe it reduces male aggression towards post menopausal females. etc etc.

the question imho is coming from the wrong direction. there’s also nobody questioning why males live past female reproduction age, since too many males clearly is a problem in primates.

so we have a species with sexual dimorphism, similar life expectancy for the sexes, high age at first birth, offspring remain dependent for a long time, male aggression is pervasive, polygyny, gendered social roles, yet we only ask „why do females outlive their fertility“?

it’s like asking „why are female ants not all fertile?“ disregarding that social order is more important to ant survival than reproductive capacity of individual members.
but yes let’s avoid the topic of male aggression like the plague.