The prevention is deterrance: because you need to buy the ticket up front, ticket inspectors can check that you have the right ticket and issue fines if you do not. You can cheat (for many journeys you can walk on and off the train without passing through a barrier, especially at odd hours) but you are taking a risk. If there's no pre-commitement to the journey then there's no real deterrent risk.
Is there a person or machine who physically inspects the ticket before you board each train? Or is it that the person who sold you the ticket is probably going to be able see if you physically go to the wrong train for the ticket you just bought and yell if you go to the wrong one? Or is it that for most folks, the act of talking to a physical person keeps them honest even if they could lie and buy a cheap ticket but board an expensive train? Or something else?
The key difference is that the paper ticket specifies the entry and exit stations.
When tapping a smart card you don't specify the exit because that's exactly the thing that makes it a faster/more convenient method of payment. There is no way to check whether you intend to pay until you exit, and then it's already too late unless a physical barrier is present at every station.
There is usually a machine that inspects the ticket on entering the platform. There is usually a machine that inspects the ticket on exiting the platform, and there is sometimes a person walking along the train asking to see everyone's tickets. The machines are backed up by people, if there's no-one manning the line of barriers then they're just left open, and they act as a fallback for damaged tickets and the like, as well as someone you can plead your case to if you've lost it. Generally if you try to enter without a ticket you'll be turned away. Trying to leave without a ticket will be harder without paying a fine, as will getting caught without one on the actual train (again, you can try to plead your case, and there are legimitate reasons like the ticket machine at the station you left from not working, in which case you can buy the ticket at your destination, but you'll need to be believed).
