The reason is, *GSM' is that it is a form of institutionalised ageism, an attitude that just dismisses us, our concerns, our existence as autonomous individuals just as much as racism, sexism, ableism. It is a dismissal of 25% of the population as people of no account.
Have you read the language Boris Johnson was using about older people, as a group, as it was reported to the COVID inquiry today.
I do not get upset about it, I cannot be bothered to waste the emotion on it, but I have always stood up for any group that is being marginalised, whether women, other ethnic groups or any other group and ageism is insidious throughout society and if we do not stand up for ourselves who will?
Using the phrase 'grans' as a generalised term for older people is almost always perjorative and a way of stripping older people of instrumentality in their lives.