There's tricks you can do with sensors. Nissan's smartkey system detects when you're inside the car, for example.
I think it's worth noting that all the car systems that I've been exposed to work really, really well. Not even sure how they've gotten it so precise. The ones that come to mind immediately are Toyota/Lexus, Nissan/Infiniti, Volvo, and BMW.
There are some systems that require the key to be in a slot that I would assume sees its presence in much the same way as a typical modern car key. In the case of the latter, there is an antenna around the mechanical lock cyl that picks up the 'chip' in the end of the key.
But for most, the fob must simply be inside the car. If it's in the passenger cabin, the car starts. Hold it a foot outside the door and it doesn't. That's all it takes...you can set the fob in the door pocket, and the car will start...with the door closed. But not when it's open. The auto unlock functions are similar- key in your pocket, door unlocks when you touch the handle. Key in the pocket of someone standing an extra step away...nope. It's pretty cool.
With cars, the reasoning is pretty simple...a lock cylinder is without a doubt the easiest part of a modern factory security system to defeat. I liked when Volvo came out with the key that was just a remote with a little plastic nub on it...it said, hey, this metal key shit is dumb; you can easily defeat it, but have VERY little chance of enabling the rest of the car without the presence of a programmed transponder key.
So what I wonder, is why exactly house keys ARE so simple in an increasingly paranoid world. Hell, even a cheap older car with a plain steel key is more complex...it's a lot harder to defeat even shitty cylinders with pins on both sides than it is most household locks with pins on one.
The answer is probably that there's just no reason. Car doors (assuming there's an actual lock cylinder there) still open with a plain, unprogrammed key. Why? Because it's already easier to just smash some glass than it is to defeat the mechanical lock. The same is true of your house; and since your house is not at risk of being driven away, what's the point of a 'smart key'? It's just a convenience with much more risk than reward. Forget computer-aided hackery to break into a house...all you'd need to do is stick a couple wires together to trigger the door's lock.
Cliff notes: you can't 'hotwire' a modern car, but you could hotwire a lock on your front door.
