It may have been happening for thousands of years, but things have changed a lot in that time. We don't tend to live in large extended family groups anymore (well, some cultures do, but that has its downsides too) so providing care is more tricky than living in the same house. Often grandparents are older with health issues due to many people delaying having kids until their own lives are sorted, or they're young enough that they're still working a lot so might not have the time or energy for it. Some people who in the past might have felt obligated to help likely feel more empowered now to say 'no thanks! I've done my child rearing' instead of doing it out of a sense of duty.
We're ever moving towards a far more individualistic society and that has pros and cons. I see multi generational households amongst my extended family and in some ways it's helpful, having more adults around to help, but in other ways it isn't (more interfering, more housework, often women are expected to take over professional level care of the elderly in return for them helping with kids when they're little). I also see situations where grandparents provide care and it is overall not that helpful, strings are very much attached, they do things very differently then get aggrieved when asked to do it the parents' way. There are some grandparents who help out and are run ragged, then the parents are confused why they're not jumping for joy with news of a third child. There are cases where it works well and the parents are happy with the care provided and the grandparents love doing it and are able to and that's great, but imo lots of people expect that to be the case when actually it takes many colliding factors for it to work out well (time, health, money, location, quality of relationship, work to name a few).
I don't think it's entitled to ask for help, that's okay as long as the askee can decline. I do think it's entitled to expect it and then complain when it isn't provided. But that's me coming from my culture which is very individualistic (which I love), I know I chose to have my child so the responsibility falls on us and nobody else is obligated to help out. If help was offered that'd be nice of course but in our case it isn't and I feel no resentment over that even though it's bloody hard at times! None of our family have ever watched our son for any length of time, we've had two nights out in three years and both times my best friend housesat after kiddo went to bed. We pay for every minute of childcare we receive and that's fine, he's our child and our responsibility.
It's bloody hard work looking after a child and I understand why someone in their sixties or beyond might want to have a loving relationship with their grandchild but with no obligation to provide proper care, I see some parents complain that the grandparent only wants to spend time with their child rather than babysit them which I do think is entitled.
Before you had a child did you ask about future help? If you did then it didn't materialise I can understand being disappointed, though I'd caution anyone not to have kids assuming there'll be help because anything can happen.
There are many ways that society can function and up and downsides to all. I'm sure many people who want help with their kids from grandparents might balk at the idea that when those grandparents are old and infirm, possibly with dementia or physical health needs, the would be expected to care for them in return. Pros and cons.
This conversation often focuses mostly on grandmothers too, who tend to disproportionately provide care compared to grandfathers. Some women who've done their time raising kids might be glad to have moved on from that and in retirement finally enjoy their life and hobbies as they see fit, unencumbered by childcare responsibilities. It does make me sad when people look down on and judge that, as if spending years raising a family isn't enough, you have to continue with it all over again or you're deemed selfish. My parents don't provide any childcare, they're having a blast in retirement travelling, doing fun things and hobbies, and I'm glad for them. You only get one life and when you get older you become acutely aware of the time you have left. If you don't enjoy running around after small children, despite them being grandchildren, you shouldn't have to just because the parents had kids they can't handle without outside input.