Thieves might gain entry through a small window, wall cutout, escape tunnel, whatever.
But once they're inside, they can just unlock a door and walk out. With your bulky TV, if they like.
Depends on the deadbolt. Some (such as mine) require a key on both sides. Since this is a safety hazard, we keep a key hidden close to the door. You might look directly at it and never see it.
"...Dumb all over, A little ugly on the side... "...Frank Zappa...