There is no doubt in my mind that I was strictest on the oldest of my now three grown-up children. I put this down to one thing: being a 'learner parent.' The second got it easier, and the third easier again.
The point is, we aren't trained to be parents. We pick up on how we feel we should behave from our parents. We carry forward a lot of their faults and shortcomings emulating the they brought us up. I expect it will be that way for a long, long time. And our parents, of course, picked up their ideas of how to behave as a parent from their mother and father.
Maybe one day there will actually be some education delivered to all upcoming parents. For example, compulsory lessons for both and impending mother and father when the woman learns she is pregnant. We know, of course, that some men and women do take to making the effort to learn what will be expected of them once they become parents. However, these are in the minority - a very small minority. In the meantime, most of us will simply blunder on.