Why are glioblastomas so common and is there anything a person can do to reduce the chance of getting one ?

Uncommon cancer: You have misunderstood something you read. Glioblastomas are relatively uncommon cancers, with about 12,000 new cases each year in the entire US. Not exactly rare, but that's 20 times and 15 times less common than breast cancer and prostate cancer, respectively. Most other cancers also are more common than glio. There is no known prevention against glioblastoma, so good thing it's pretty rare!