The short-campaign-as-normal thing is IMO a direct result of WotC marketing* in the 3e era (and since), as short campaigns are extremely likely to lead to more demand for books and materials. WotC outright said, for 3e, that the game is designed to go from start to finish in 18 months give or take; and people took that to heart and (sadly) normalized it. WotC sold more books, and then watched and learned as Paizo refined the process into the single-path-as-whole-campaign model, leading to the current-day corporately-mandated "normal".