Most cars have floor consoles anyway so the benefit of a flat floor would be lost up front, and nobody cares enough about rear seat passengers to design a whole car around them. Secondly with a floor tunnel they have a platform that can be converted to AWD if or when that time comes.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma