[BucketsMcG]

All sorts of reasons, but it's got a lot to do with packaging and handling. Generally you want the weight distribution of the car to be 50:50 front:rear for optimal handling.

A car will handle great if you can put the engine and transmission somewhere towards the middle, but obviously that isn't great news for passengers. Mid-engined cars exist, of course, but they're usually two-seater sports cars where space and comfort are lower down the list of priorities.

So you're left with putting the engine somewhere towards the front, or somewhere towards the back. Some manufacturers, like BMW, achieve ideal weight distribution by mounting the engine relatively far back in the front of the car, and putting the transmission and some other heavy bits at the back. This is elegant, but too complex and expensive for your average runabout.

With a front-engined car, you can safely put some of the engine forward of the front axle. Doing so means more space in the passenger cabin, but hanging the weight out front makes the car nose-heavy. In an average front-wheel-drive family car, this isn't necessarily a bad thing, and might even be desirable, as it means good traction and a tendency towards boring, predictable understeer.

Sling the weight out back, like a Porsche 911, and you get the opposite problem: the car naturally tends to want to swap ends in corners. Some drivers find this fun, right up until it kills them. If a 911 is a safe car today, it's because Porsche has spent decades pigheadedly perfecting a layout everybody else abandoned long ago for its tendency to send you backwards through hedges.

[spindle]

I like this reply very much except that I wonder why people say that sports cars are uncomfortable. I find all the ones I've driven, including two mid-engined ones, super comfortable. Maybe it's just me. (In any case, your point that they're not space-efficient stands.)

[sliken]

Super comfortable mid engine ... Cayman? If you give up the rear seats it does help quite a bit, but does limit your market.

[CobaltFire]

BMW does not use Front Mid mounting on anything current that I’m aware of, and I don’t think they’ve ever used a rear mounted transaxle (except the M1).

Alfa 75’s are along the lines of what you said, but not any of the Germans.