Let me give you my perspective as a parent who fully intends to give every possible educational boost to his child.
I have a job, and lots of other obligations. Quite frankly, I don't really care if she wins the science fair or not. I suspect that if she decides to compete when the time comes, she'll have a good shot at it just because of the level of effort we put into education and the equipment we have around the house. The majority of my free time and money is spent on this, so we have a lot of unusual stuff.
It's easy to put that kind of effort into your own kids, because they live with you. They're around for all of your spare moments, and they can use equipment you have access to that demands close supervision. Now here's the part that's relevant to your comment - I'm also heavily involved in her school, trying to do as much as I can for all of the other kids who are interested (the topics are computer science, math, and robotics). I get to spend maybe two hours a week working on that, after my other obligations are taken care of. It's simply not possible for me to do more for the school, and even if I could the problem is concentration of resources. Given a certain budget (time and money) that I have available for this project, it's obvious that I can make a large difference for a small number of kids, or a small (forgettable) difference for a large number. Taken to the extreme, I can make an enormous difference for one kid (mine). No matter what I try to do, I'll never be able to make that kind of difference for even a small classroom full of kids, and certainly not a whole school. So I'm left with this choice: Do I spend my resources in a pursuit that's likely to be ultimately fruitless (i.e. trying to "raise all boats"), or do I spend it on something I have a very high chance of success with? Which of those is more wasteful? My answer is that I do the best I can for the school, but I feel no guilt about giving more to my own kids.
In the end, if my daughter ends up winning a science fair it's going to look exactly like a case of inherited privilege, and maybe that's what it is. That's not because I'm trying to hold down everyone else though, it's just a simple matter of logistics. If I could give that kind of boost to everyone, I'd do it in a heartbeat.