Because knowing about the presence of a condition is better than not. Depending on the severity, untreated ADHD during the years of life where a child begins to establish good study habits, management of the condition, and other tools that work for them, can lead to issue down the road and into adulthood. We have the ability to address conditions like dyscalcula with little interventions to help the student be successful.
Just because something is imperfect doesn’t mean it should disregarded completely if the benefits (academic, social, and career success) outweigh the drawbacks of being untreated. The stigma argument is just FUD and letting that take over decision making for the well-being of a child is a bad path to go down.
There are often, unknown to the parent, invisible scars that the child with a non-neurotypical condition will carry for the of their life after having found out about a condition they’ve had since birth and was not addressed during the most critical time of their life when early treatment could have greatly reduced the harm caused by this disorder.