But having a car stolen is way less of a problem than all your personal data being stolen. If my car is stolen because someone stole my bag with the keys in it then all I lose is my insured car.

If my laptop is stolen with the 'keys' to the laptop also in the bag, my life could become very difficult very quickly taking me years to recover from.

I think the point is you don't want to have the same paradigm as car + keys when it comes to your private information.

Do you use full disk encryption? A lot people don't and it's easy to get your data when you got logged in as another user.

I use FileVault built into OSX.

the change of you having your car keys and your car in your backpack at the same time is much, much lower.