Also, it actually make sense to want the kid to go to sleep sooner then adults and also to go home for dinner or whatever.
Or just don't care lol. Children love pushing boundaries. Sometimes it's good (learning a sport, improving a skill, taking on the bigger, scarier feature at the playground) and sometimes it's annoying (wanting more ice cream, staying up late, etc...). I'm convinced it's just part of growing up.
> it actually make sense to want the kid to go to sleep sooner then adults
Yes, but it isn't enough if you know that, the child has to know that, and actually agree with the reasons. If the reason is "young people need more sleep, you will be so tired tomorrow" you can make the child see that reason (let them stay up once, show the consequences, hope it doesn't backfire). You still get push back, but not for it being unfair. If the reason is that the parents want time for themselves then the child is right that it's unfair, and the better solution would be "alone time for everyone. You in your room, we in the living room, internet shuts off at 9". Similarly "we eat dinner together" is a fair rule as long as everyone is held to the same standard. You still get push back, but a fair rule is much easier to enforce than an unfair one.
As a child, I often read until 2-3am, and was just fine the following day (wish I could do that now). Any actual enforced attempt at going to sleep would have felt unfair because it'd have been earlier than I needed.
What my parents actually cared about was that I was in my room, and that I got up without being cranky or letting it affect school the following day.
It's much easier to "sell" the child on that, by being honest about wanting quiet time and wanting to make sure the child isn't too tired, than convincing them a bedtime they will have secretly violated multiple times without feeling too tired is reasonable.
So from pretty early, my "bedtime" was "alone time" as you suggest, with the proviso that if I couldn't get up without being prodded in the morning that was evidence I was taking the piss, and the consequence would be tightening up of the actual bedtime for a while. That felt fair, and honest, and the rules were understandable, and meant I often stayed up way later than my parents would have thought was reasonable, but since it worked, it was fine, and I self-regulated.