The cheaper insurance has reasons. Where did you get the 3.50 from?
3.50 is a colloquial expression for = the argument doesn’t hold due to insignificance;)
But I recalculated it for you: with HUK for example, you pay exactly 11 euros more per year if the car is not parked in a lockable garage. However, the insurance terms state that it must have been locked as well - you have to prove that.
I fully agree with the argument that the garage is more of a utility space than it adds value for the car. Another neighbor, for example, has an old 3-series Touring (probably still worth around 5-6k euros) - the car is parked outside - while inside he has a workbench etc. then your theft argument applies if it’s about tools, for example.
Most people I know don’t use the garage as a garage, but as a utility space for tinkering. As mentioned above: it’s a matter of type - almost no one does it because of the car.