Because,
None of the medical tests currently used to detect brain damage in humans have found harm from marijuana, even from long term high-dose use.
An early study reported brain damage in monkeys after six months exposure to high concentrations of marijuana smoke. These tests aren’t to be taken serious, because these monkeys were tested with outdated technology thus making the result inaccurate.
In a recent, more carefully conducted study, researchers found no evidence of brain abnormality in monkeys that were forced to inhale four to five marijuana cigarettes every day for a year. This clearly proves that marijuana leaves NO brain damage.
The claim that marijuana kills brain cells is based on a report dating back 25 years and has never been supported by any scientific study.