Why is ADHD called a disorder?

It’s considered a disorder because of the last D.  It is a measurable impairment of executive function and working memory.  I consider it a neutral Superpower where depending on the situation can be an obstacle or advantage.  That’s just me.
I don’t enjoy my ADHD symptoms sometimes.  I try to minimize that time by creating situations and goals that invite ADHD traits to help me out.
An example is using my stubbornness to work through problems that people have given up on.  Then the trust I have cultivated in my hyperfocus and my scattered thoughts helps me arrive at innovative solutions in non-linear ways that others could never imagine.
I hope you find some enjoyment of your ADHD.  It’s the best roller-coaster ride I’ve ever been on!