As stated in a thread above, while it is not directly profitable for drug companies to cure diseases, it is profitable for their insurance agency cohorts. The current system does not totally dis-incentivize cures.
A friend of mine works in pharmaceutical marketing, and the big revelation I got from him was that the marketing departments run the drug companies from the top to the bottom. If marketing doesn't see a billion dollar market for a new drug, it won't even be considered for clinical trials. The VC in biotech knows this, so they won't fund startups that don't have this potential.
There was a HN thread on this recently in relation to antibiotics. [1] Antibiotics are a special case for their own reasons, but ultimately if the potential for insane windfalls isn't there, big pharma is not interested.