See, if it were dollars, I could just reach into your bank account or your mattress, or wherever you keep them, and take them. Can't do that with bitcoins.
That's just out of practicality.
Yes, in that it's impractical to calculate between now and the heat death of the universe to get a 50% chance of guessing the right private key.
Just like would be the case if i hid my coins and encrypted the location...
In both cases the only protection is a piece of information that exists somewhere.
And someone could steal the information.