I think therapy can be wonderful, life changing, even life saving for many. I think most people do benefit positively from it.
That said, people with narcissistic personality traits tend to be good manipulators, and it is said that they are also quite good at manipulating therapists. I have watched this happen, and it was quite astonishing how quickly the therapist was tricked into believing a sob story. Many narcissistic people thrive on sympathy and victimhood. They can paint some quite believable pictures and a sympathetic therapist may inadvertently feed into these self-obsessive behaviours. I'm pretty sure most narcissists don't go for therapy to be cured of narcissism, but that doesn't mean they don't go for therapy. In my experience, as soon as the therapist cottoned on to the narcissistic behaviour and challenged it, the person simply switched therapist.
And even if we avoid the narcissist label, I can think of a few quite self-obsessed people I know who treat others poorly but use therapy-speak to validate their shitty behaviour and demand special treatment.
So I do see where the OP is coming from.
Edited to add: the person in my experience was diagnosed as a narcissist by a qualified professional, so I'm not throwing labels around without understanding their meaning