While a misdiagnosis of having parasites when you don't is possible, it is highly unlikely. It is more likely that one be misdiagnosed as not having them when they actually do.
An old saying goes "proof is in the pudding" and has a lot of value. If you examine your stool and you see what appears to be parasite on a repeated basis, then it is likely that you have parasites.
Why do people struggle with getting rid of them? The difficulty lies in the fact that these parasites can be very hardy and can outlive us. We are also constantly reinfected from our environment, especially from our pets and other family members.