Why is this actually the case? For what reason, when it is presumably significantly more cost-effective than a knee wall?
A knee wall is the exterior wall on the eaves side of a house, built above the rough ceiling of the top floor, on which the roof structure rests. It is a typical feature of purlin roofs. The higher the knee wall, the more usable floor space is available under the roof slope. Source: (exceptionally from me) Wiki because by cutting it back, much floor space in the attic is lost. However, if you need to accommodate a considerable amount of usable rooms at the top, the ground floor area would in turn have to be chosen larger. Nowadays, some plots no longer allow this (cost, size, availability). Also, this may result in an imbalance of rooms between the ground floor and the attic (too many downstairs, too few upstairs).