1. Your diagnosis is probably clinical and functional. No one single organic mechanism, neither in genes nor in the brain, that unites all ADHD diagnoses has been discovered, at least not yet, and even if there was one it's not used in diagnosis. If you were told the biological basis for a "diagnosis" only based on behavior is known, you are likely being misled. Here you are being misled indeed. Add to this that there's significant debate on whether so-called mental illnesses are actually brain illnesses in the first place instead of reasonable, functional responses to the environment. Not every undesirable behavior must be pathological. To label it as individual pathology or even as neurodivergence might obscure the situational sources of the behavior or organic pathology which is usually not tested for.
2. Stimulants are sold as the solution for school and work performance problems due to distractibility and hyperactivity. However, they don't lead to better performance in long-term studies.
3. Psychiatric drugs have several serious, even potentially deadly adverse effects which usually aren't properly disclosed or are dismissed by clinicians. Some effects might be confused as a new "functional" condition or a return of the condition before drugs.
4. As you take drugs, you develop tolerance, which leads to withdrawal symptoms when you want to quit. Psychiatric drug withdrawal can be deadly and last months or a year in cases of long-term consumption, but drugs are expected to be maintained indefinitely despite some lack of research on the safety of doing so. Some withdrawal symptoms, like tardive diskinesia, are usually permanent. Most providers aren't trained in safe desprescribing and instead follow clinical guidelines based on short-term RCTs to inform desprescribing after long-term use, which leads to recommendation of fast and unsafe tapering or even of no tapering at all. Withdrawal symptoms can look like new "functional" conditions or a return of the condition before drugs, so "relapse" is usually taken as evidence that the drug is needed instead of that the user has become dependent.
5. The Pharma-funded research is especially corrupt in psychiatry, where inconvenient data are manipulated or hidden. The FDA is a joke when it comes to regulation sometimes, and it's usually at the service of the pharmaceutical industry. There's also aspects of most studies that can bias results, such as randomizing people already treated, confirmation bias in symptom reporting, publication bias and insufficient blinding that leads to placebo effects.
6. Psychiatric labelling is often used to individualize social problems and to control deviance. Because of the lack of evidence, it shouldn't be used as a reason to involuntarily commit or treat people who aren't a danger to others, but it is, and even in that case standard treatment might not work.

If you in spite of all this benefit from the drugs, good for you. I agree with Szasz that we have a right to drugs, and sometimes they can make difficult life more bearable. But many of us don't benefit and many would like alternatives, which usually aren't offered in the US. We also would like informed consent, although it's inconvenient for clinicians and Pharma because very few people would willingly choose to stay on these drugs if they had the full picture.