The symptoms of CTE are similar to or the same as other neurodegenerative disorders like Alzheimer’s or mood disorders which are also currently difficult/not possible to diagnose using clinical tests. Currently the only for sure determining factors can be observed by examining the brain tissue which can only be done by autopsy.

I have no doubt there is active research into better ways to diagnose CTE, but like many neurological problems it’s difficult.

From the first Google result for your question:

"A diagnosis requires evidence of degeneration of brain tissue and deposits of tau and other proteins in the brain that can be seen only upon inspection after death.

Some researchers are actively trying to find a test for CTE that can be used while people are alive. Others continue to study the brains of deceased individuals who may have had CTE, such as football players.

The hope is to eventually use a range of neuropsychological tests, brain imaging such as specialized MRI tests and biomarkers to diagnose CTE. In particular, imaging of amyloid and tau proteins will aid in diagnosis.

"Researchers are actively working to develop PET markers to detect tau abnormalities associated with neurodegenerative disease in people who are living.

Researchers are also working to develop tracers that target and bind to tau buildup and other proteins on PET scans. Studies are underway using these types of scans and tracers to look for tau buildup in the brains of retired athletes who experienced head injuries."

Chronic traumatic encephalopathy

With our current technology, you need to take a piece of brain and run tests on it to diagnose CTE—the symptoms are too similar to other types of dementia to differentiate CTE in any other way. You can’t take a piece of brain without doing serious damage, so doctors don’t do that to living people.

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I’ll just add to all of the great comments by saying that one of the challenges is money. Take, for example, neurodegenerative disease. There is a huge amount of work being done right now to develop a blood test to screen for Alzheimer’s (Google C2N Precivity). These efforts have tons of money behind them because there are huge benefits to pharmaceutical companies when screening patients for participation in clinical trials. There is not the same kind of incentive for CTE diagnosis, and it is nowhere near as prevalent as Alzheimer’s, so you don’t have the same industry financing and inertia in CTE research.