 This phase II trial studies how well bendamustine hydrochloride, obinutuzumab, and dexamethasone work in treating older patients with diffuse large B-cell lymphoma. Drugs used in chemotherapy, such as bendamustine hydrochloride and dexamethasone, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Monoclonal antibodies, such as obinutuzumab, may find cancer cells and help kill them. Giving bendamustine hydrochloride, obinutuzumab, and dexamethasone may kill more cancer cells. 