Just like with humans, exercise is a major factor in health. Outdoor cats generally get way more. Another factor for this observation (that I also made) could

Just like with humans, exercise is a major factor in health. Outdoor cats generally get way more. Another factor for this observation (that I also made) could be genetics - generally "pure-bred" (aka more likely to suffer from disease linked to genetic factors) are more likely to be strictly-indoors while outdoor cats are more often "mongrels".