Airlines attach an ID tag to each item of checked luggage. On a sampling basis an invigilator located at the exit from the luggage carousel room, before Customs, matches the ID tags to the possessor of the luggage. Airports are crawling with police and surveillance cameras, and that is good.
In Latin America, IME, most inter-city bus lines attach ID tags to each item of checked luggage, and the driver or his assistant matches each luggage tag to the person who is recovering the luggage. One usually cannot just take a bag and walk away with it. However this vigilance does not necessarily occur in other parts of the world.
Also, deliberately stealing someone else's luggage and then taking it through Customs would be very risky: how would one explain the presence in the stolen luggage of prohibited sausages or protected species or illegal drugs or unacceptable images or ...
Well, the thief could claim that s/he took the bag from the carousel in error.
To eliminate that excuse, and to help me quickly find my own luggage, I have my initials paint-penned in large letters onto my luggage and the my-body-facing side of my packs.