Back Seats Are Just for Show

"We need a four door car! What're ya' gonna do about dat, buddy?"

—A friend of a man who you need to take to the hospital, Mafia

A video game trope where cars with two doors are only able to carry two people. Never mind if it is a four or six seat hardtop, or even a two door station wagon/SUV. It appears that the back seats are just for show. This trope makes it that if you even need to carry more than one other person, you need a four door car.

The reasoning behind this trope is basically developer laziness. It's extra work to include coding and animations to show a passenger get in the back seat through the front door—a lot of work for a situation that is very rare in the games it can show up in.

Note: This trope can only apply to a game with vehicles where you need to carry two others at any time—it is not this trope in games where you would never need to use the back seat in the first place, two-door car or otherwise.

Examples of Back Seats Are Just for Show include:

Both Saints Row games avert this partially in that two-door four-seat convertibles can have rear passengers (they hop over the side), but two-door four-seat coupes can't, using this trope. Unlike other titles in the same genre, if you have more followers they will independently hijack enough cars to carry themselves and follow you rather than be left behind.
The Saboteur works the same as Saints Row.
Pick any Grand Theft Auto game. Most annoying in Grand Theft Auto: San Andreas, since drivebys are an important mechanic. The number of doors equals number of passengers rule also applies in a broader sense. Trucks, semis, and cargo helicopters only have one passenger door, so only one NPC can ride along. The big business jets can't carry passengers at all, and neither can any boats no matter how many seats they have; when passengers ride along on boats for activities in 4, they load on and off in a cutscene. The Maverick helicopter is the only air vehicle that has four doors, and thus can carry three NPCs. Tanks would need multiple crew to operate in reality, but no one else can come along if you take one. Vehicles like the Stretch limo and the various ambulances and vans look like they could fit more, but stop at four because of the door limit.
Mafia and Mafia II
True Crime: Streets of LA
Driver: Parallel Lines and Driv3r
This is why all the taxis in Crazy Taxi are convertibles.
The Godfather and the sequel.
The Simpsons Hit & Run
Scarface: The World Is Yours