True, but that can definitely be influenced in utero. For all we know something like the microplastics we now all have in our bloodstream might cause the brains of fetuses to develop differently. Also there’s a genetic component. What if people with ADHD are currently more likely to have children now than a few decades ago.

I’d argue there’s many potential reasons why there might be more people with ADHD now. I’d be surprised if the rate is fairly constant. Not that that’s easy to measure due to changes in how we diagnose (as others have already pointed out).