Why would "someone just living their life and doing all they already can to get by and not worrying about these things" be seeking a diagnosis/treatment in the first place?

Things are generally only considered "disorders" when they prevent you from leading a normal life, with ability of advancing in a career, forming meaningful relationships, etc. For example, Trump may well exhibit the symptoms of Narcissistic Personality Disorder, but it clearly does not get in the way of his life, and he's able to accomplish goals that he sets out. Thus, he cannot be said to have a "disorder".

Also, for many of us, psychiatric diagnosis and treatment isn't "costly", it's free. And if you meant opportunity cost, patients generally have the freedom to stop treatment if they feel that the costs overweight the benefits.