Think about it. If your bike gets nicked out of your car, you'll have no bike, and a car that's likely to need to go to the bodyshop (from general wrenching of doors etc). You'll lose your NCB, and have no transport at all for some time.
Would really make having your bike nicked into a massive kick in the balls.
If my bike got pinched from the garage I'd have to get a replacement garage door (also hundreds) and my garage woudl be left unsecure for days until it got replaced, leaving thousands of quids worth of other stuff pinchable. I'd lose my NCB on the house too.
Conversely, with the car you can have protected no claims or repair windows and doors yourself fairly cheaply scrappy replacement.
Depends on your car.