Interesting point.
Hard to check every commercial prop airframe design ever certified - but it seems as though the only "basic" propliner with seating in line with the props from the get-go is the ATR 42 (one row).
The original versions of the DHC-8 and Fokker 27/50 have the props just (barely) ahead of the seating - but in stretching them, seating moved into the prop plane. As also the ATR-72.
Could be an oversight, or could be someone decided a prop separation was a low-probability risk. At least against the return from all those lovely extra revenue seats.
(Lots of designs have cabin space in the prop plane, but devoted to galleys, bogs, bulkheads between first/cattle class, etc. If you choose the wrong moment to get your coat from the stowage closet on an F-50....)