Because diseases are complicated and your body’s response involves a number of factors. We know that there is a large group of people who had no known health conditions prior to some number of COVID cases, at which point they did have a chronic condition, and tests have failed to link it to other known diseases. This again is similar to other things - for example, arthritis has some components like age or genetic predisposition but also risk slog goes up with things like obesity, smoking, exposure to certain chemicals, or bacterial / viral infections in and so there isn’t a simple cause or effect relationship because for any given trait you can find people who have it but did not develop the condition or vice versa. What researchers have done is find correlations in large groups to identify how the many factors increase your odds of developing the disease - even if most people don’t develop it, we have a large enough population that many people will.
Long COVID is similar in that they have identified risk factors like age or not being vaccinated and have some theories for the underlying mechanism and biomarkers which hopefully will make it easier for clinicians to identify patients or warn people with high risk factors.