There are things I think everyone wonders about their parents decisions, and why they did them. I'm sure my children, and yours will be the same. If you ask, it may have been so insignificant to them that they can't remember the reason.
It may well be that your parents thought you loved going to your grandparents so much. It may be that they had something they did on Saturday nights that made it convenient. It may be that it was just something that had always happened, so it continued because no one called a break to it. It could be that they felt it was necessary for you to have a close relationship with the grandparents. it could be that one of them had always done it and loved it. Or it could be that just your parents appreciated the night to themselves. We can't tell.
Just an example of how there could be lots of reasons which you wouldn't necessarily know. I'm not sure it's helpful to start wondering why and why not. There's probably other times that you don't remember where they were child centred, but because it was then a non-issue to you, you haven't remembered.
Remember when you judge your parents by todays standard, your children will judge you by their day's standard.
I've had times with my own dc (now adults or nearly) that they've asked why something happened. Often it's been because as far as I knew they wanted to do it. I can think of two things that we discussed fairly recently.
DD2 said that I let dd1 and ds give up music lessons, why hadn't I let her give up the trumpet. I said that I hadn't known that she wanted to give up the trumpet, and if she'd said she wanted to then I would have let her. I also pointed out that dd1 gave up when she went to sixth form (dd2 stayed at the same school for 6th form) which meant that she wasn't home until after 5pm, and her lesson had always been 3:30pm and the teacher didn't have space, so we'd have had to change teachers. Ds gave up when he was quite badly ill for a year, so both had specific reasons to give up at that point.
She then said that she hadn't wanted to give up anyway (so not quite sure why she asked), but she'd have liked to be asked. She still plays so it obviously didn't traumatise her that much.
Another one was when dd1 was 8yo she had pneumonia. Her symptoms were entirely a sore shoulder, so I thought she'd pulled a muscle. We got halfway to school (40 minutes away) when she sat down and went dizzy.
She asked me why I'd been walking her to school that day. Her memory was feeling really bad as she walked back up the road.
She has no memory of crying when I suggested we got the doctor to check her shoulder before we went to school (I still thought it was pulled muscle at that point) because there was a fun event at school. Telling me that her shoulder was feeling better, and she felt really lively etc. In fact when I suggested I drove her to school she said that she wanted to walk.
Both of those were decisions I made with the information I had. The former I'll hold to it not being a wrong decision, the latter it was a bad decision, but made in good faith and not out of any ill intent, and dd1 knows that.
Decisions are made and sometimes regretted. I can think of some odd ones of my parents. I'm sure we all can.
For my parents, they're the opposite of risk takers. They'd panic about me walking down to the village on my own in case of cars/accidents/nasty people. No, we didn't live anywhere with a particularly bad reputation.
If I wanted to go anywhere with friends, even friends with parents, dm would assess and decide if it was safe before she'd allow me to go. They'd be times when I was stopped for doing something because "the bus might be late and then you might be waiting at <in a very populated area which had no specific issues>a bus stop when it might be getting dark" so all the possibles adding up into a potentially small risk.
But on holiday we used to go to a very rural area. We'd travel miles further into the rural area for their chosen beach. We had to park in the car park (aka field) walk a couple of miles down to the top of the cliff, scramble down, and then walk to the far end away from everyone else.
That was their decision.
However if there had been an incident, and dm was more than capable of imagining all manner of unlikely events most of the time, then to get help would have involved a 20-30 minute clamber back to the car. 10 minute drive to the nearest phone box, assuming that one was working which was probably at best 50/50. Even when they got mobile phones there was no signal until you were practically back at the phone box.
Now we never had an incident. But if we had, then their choice could have made the difference between life and death at worse, and maybe I'd be asking them why they chose to go to a beach like that.
As it is, I just consider it to be a rather odd decision out of line with their normal take no risks and stay where everything is totally safe attitude.