Form over function?
The trunks of the M37 and the old Lexus GS weren't very good either. Must be a Japanese performance sedan thing. Not sure about the new GS.
Comparatively the Germans always give you a square trunk. The CLK had flip and fold rear seats and could carry 4x8 plywood in spite of using goosenecks.
Even the Z4 had a pretty damn good trunk, decently sized and quite square, and it used struts. The Jetta TDI had a cavernous and well shaped trunk for such a small car.
I suspect the Camry and Accord have better designs given their target market, but overall it is a gripe I have. The A4 for example has much more effective cargo capacity between the flat fold rear seatbacks and the giant rectangular prism design of its trunk, despite being 7 inches shorter than teh TL. Rear legroom not so good though.