A third row looks like an addition, but it changes the packaging of the entire rear of a vehicle. The compromises land in places buyers do not examine until they own the car.
The seats sit in the least useful part of the vehicle
The rearmost row occupies the space above and behind the rear axle, where the floor must clear suspension components, the exhaust and often a fuel tank.
That raises the seat base relative to the floor, leaving occupants with their knees high and little support under the thighs.
Headroom is constrained from above by a roofline that has usually started to fall, so the space is limited in both directions at once.
Luggage capacity is the direct casualty
With the third row upright, remaining boot space is frequently reduced to a shallow area behind the seat backs, sufficient for shopping rather than for luggage.
A vehicle carrying seven people is precisely the situation in which luggage is needed most, which is the central contradiction of the format.
Folding the row flat restores the space, but the folded seats occupy floor depth, so a seven-seat version usually has a higher boot floor than the five-seat equivalent.
Crash engineering has to be reworked
Rear impact protection normally uses the space behind the rear seats as a crush zone. Occupying that space with people removes the deformation distance.
Manufacturers respond by strengthening the rear structure, adding cross members and revising the fuel tank position, all of which adds weight.
Head protection also has to extend rearward, requiring longer curtain airbags and additional anchorages that the five-seat body does not carry.
Access requires a mechanism that costs space
Reaching the third row means the second row must slide and tilt, which requires runners long enough to create an opening an adult can use.
Those runners consume length, so the second row sits further forward than it otherwise would, reducing its own legroom to serve the row behind.
Sliding second rows partly resolve this by letting owners distribute space between the two rows, which is a compromise made adjustable rather than removed.
The weight penalty follows the vehicle everywhere
Seats, mechanisms, structure and additional trim add mass that is carried on every journey, including the great majority made with the row folded away.
That weight sits at the rear, which affects balance, and it reduces payload available for everything else the vehicle carries.
This is why the seven-seat variant usually posts worse consumption and slightly duller responses, and why owners who rarely use the row often conclude it was not worth the compromise.