The Bernese Mountain Dog's expected lifespan is among the shortest of any breed on this site, and unusually the explanation is specific rather than general.
Size alone does not account for it. Plenty of dogs in the 70 to 115 pound range live eleven or twelve years. What sets the Bernese apart is histiocytic sarcoma, which appears explicitly on its health list as histiocytosis. It is an aggressive cancer arising from histiocytes, immune cells found throughout the body, and the Bernese is more affected by it than any other breed. It can present in the spleen, liver, lungs, joints or bone marrow, it spreads quickly, and it typically strikes between about five and eight years — the middle of a dog's life rather than the end.
The signs are non-specific, which is part of what makes it hard: weight loss, lethargy, loss of appetite, lameness in the joint-associated form, or laboured breathing. There is no screening test in routine use.
What a buyer can do is choose the breeder carefully. Ask about longevity in the line — what age the grandparents reached and what they died of — and treat a breeder who tracks that as a serious one. Some breed clubs run health surveys and registries specifically because of this cancer, and a breeder engaged with them is a meaningfully better bet.
The rest of the list runs hip and elbow dysplasia, bloat, degenerative myelopathy, progressive retinal atrophy, Von Willebrand's disease, allergies, arthritis and cancer generally.
The Pit Bull's 12 to 14 years reflects a list that is broad but not dominated by a single killer: hip and elbow dysplasia, skin allergies, heart murmurs, bloat, obesity, cruciate ligament disease, arthritis and cancer.
None of this is a reason not to own a Bernese. It is a reason to know the number before you fall in love with the puppy.