There are a number of perfectly sound reasons for doing this – as for example when you are working on the engine and need it to be running to check/adjust the timing.

But the main reason is – was – the presumption you're not an idiot.

That presumption no longer applies. And that's why – if you own a new car or a car that was made since roughly the 1980s – you can only start the engine if you first place your foot on the brake. Which of course requires that you sit inside the car in order to start the car.

Similarly, in a car that has a manual transmission – if you managed to find one with one. You now have to push the clutch in before the starter will engage. It does not matter that the transmission is in neutral; it does not matter that – courtesy of this "safety" device – it is no longer possible to use the starter motor to inch the car forward, which might be handy in certain low-traction situations. What matters is that you be kept "safe" – from yourself.

There are other examples. How about the way the stereo in most new vehicles turns the volume down so low you can't listen to what you were listening to – for the duration of the time the car is reversing? The way the GPS won't let the driver input a street address unless the car isn't moving (and thereby effectively defeating the point of having GPS). The way the seatbelt buzzer will berate the passenger to "buckle up" – when there is no unbuckled passenger.

Other than the bag of groceries you placed on the passenger seat.

Some hatchbacks harass you – with an endless buzzer – if you drive with the rear liftgate partially open – which is often necessary to be able to drive home with something in the cargo area that doesn't fit with the liftgate closed. Some new cars will summarily (electrically) put the transmission in Park if you try to back up with the driver's door cracked – so as to be able to see curb, etc.

This is, of course, really good for the transmission, too.