Is there scientific evidence that narcissists cannot be treated? I've often heard that people affected by narcissism cannot recover. 

I can understand that it's a long path and that a narcissist may lack the motivation and so on, but I wonder why cognitive therapy (for example) should be ineffective towards this kind of pathology.

Are there statistics on the treatment of narcissists?
It appears you have heard correctly, at least in a sense: there are no peer reviewed treatments for narcissistic personality disorder (NPD) itself. On the other hand, this does not constitute evidence that narcissists cannot be treated, either.

A review of the NPD literature by Dhawan, Kunik, Oldham and Coverdale (2010) reported an absence of evidence one way or another in no uncertain terms:


  To determine the most efficacious treatments for NPD without other comorbidities, we performed searches using Narcissis*, pharmacology, clinical pharmacology, therapeutics, and psychotherapy for reports of controlled trials from January 1980 to August 2008. [...] We identified 7 prevalence studies that had used a structured or semistructured interview, 5 of which scored 5/6 using the epidemiologic quality tool. Mean prevalence was 1.06%, and the range was 0% to 6.2%. We found no studies of treatment meeting our inclusion criteria.


The prevalence numbers refer to how many people satisfied the DSM-IV-TR NPD criteria. The epidemiological quality tool referred to is defined as follows:


  Articles received 1 point for each of the following: surveying a national population (because personality disorders are more prevalent closer to city centers, this disparity could influence locally obtained samples), obtaining a sample that was representative of the population being surveyed, accounting for nonrespondents, using a valid diagnostic instrument, having adequate training of interviewers, and assessing reliability. Assessment of the above criteria was determined after each question for each study was independently rated by each author. Then the authors met twice and came to a consensus regarding the scoring of each question.


My own review of the 2009-2015 literature did not find any new developments. There simply does not appear to be any treatment studies for NPD, efficacious or otherwise.

References


Dhawan, N., Kunik, M. E., Oldham, J., &amp; Coverdale, J. (2010). Prevalence and treatment of narcissistic personality disorder in the community: a systematic review. Comprehensive psychiatry, 51(4), 333-339.