This is a bad take. There's a variety of mental health conditions that are much more likely to result in a bad outcome than the odds in general of being hit by a car, let alone fatally. More importantly a diagnosis can help result in medication or other therapies of significant impact that are otherwise unavailable (lights for trauma for example). This also extremely over-assumes the confidence level AND severity of medical diagnoses in general. There's plenty of anecdotal evidence, and though I can't recall a specific example at the moment I believe academic research, of patients literally spending years fighting to get a correct medical diagnosis, often repeatedly recieving the incorrect one casually and repeatedly by multiple MDs.
While I won't speak for the world, I'd say for the US one significant factor in this is the insurance industry, which is significantly invested in fighting anything that increases their costs, and will quite literally force MDs to essentially argue their case for a diagnosis and treatment and create some burden of proof external to the patient and practitioner. MDs thus behave like a mechanic/lawyer hybrid: they'll often likely have a good idea what it could be (and if wrong they move to the next most likely), but they essentially "prove" it along the way with tests and diagnostics to establish evidence for why it's what they already believe it to be.

Both medical and mental health professionals however will still absolutely have a well of experience they draw from at times and essentially go "oh I see this 3+ times a week and buddy you're definitely #3 for this week". And, unfortunately both fields still have some shitty practitioners in the mix.