Who says they haven't?
"Hamadeh and associates. Chest, Vol. 94/2, pp.432-433, 1988. "Invasive aspergillosis has become a significant cause of death in immunosuppressed patients". Physicians should be aware of this potentially lethal complication of marijuana use in compromised hosts such as patients with AIDS or malignancies.)"
"Transplantation, Vol. 61, June 27, 1996. (Marijuana smoke transmits aspergillosis, a fungus having up to a 90% fatality rate if contracted by transplant patients. Researchers have strongly warned against the use of marijuana in immuno-compromised patients such as those with AIDS, chronic granulomatous disease, bone marrow transplants and those receiving chemotherapy for small cell lung cancer.)"
"Voth EA, Schwartz RH. Medicinal applications of delta 9 THC and marijuana: a perspective. Annals of Internal Medicine 1997: 126:791-8. (Marijuana is not a panacea. It is an impure weed that introduces immuno compromised patients to bacteria, fungi, and other toxic complications. We recommend sticking with predictable medical therapies and not deviating from FDA approved medicine in exchange for herbal remedies"