Because if you're saying "self diagnosis is the wrong way", but there is no other way yet, you're effectively saying you don't believe anyone who has it has it. And that isn't helpful.
As per my message: we have the CT scan/Xray to the chest showing clear signs of reduced lungs capacity following a COVID infection. Let's call this "doctor diagnosed long covid".
We also have the other self reported symptom. Because it's crazy to give it the same name, let's give it another one. We might call it the "I am feeling less well than 2 years ago" disease. It's a very common one.
Now the "I am feeling less well than 2 years ago" disease might have many origin. COVID being one of them. But considering that it has been rampant for years before COVID appeared, it's unlikely that it is the main origin of it.
According to me, it's not up to patient to diagnose it, just like we don't let patient self-diagnose brain cancer after letting them read 5 health article on Google otherwise everyone would have it - you know that joke, since why shouldn't it apply to COVID as well ? And since "long COVID" is discussed everywhere online and in the media, so it's natural people will associate with that, that does not make it true.
Because if you're saying "self diagnosis is the wrong way", but there is no other way yet, you're effectively saying you don't believe anyone who has it has it. And that isn't helpful.