When I book an airline ticket, I'm making a contract that says "I'm paying you to transport my person and belongings from Point A to Point B to Point C with these details." Now unless I cancel or modify that reservation, the airline is expecting me to follow through, because I'm monetarily invested.
What are the consequences of missing my flight? My name will be on the manifest and the airline will wonder what happened to me. They probably won't file a missing persons report, but their records will note that I had a flight but I wasn't on it.
So what if I've done some "hidden city ticketing" and the aircraft on the last leg crashes? My friends and family, and first responders, might wonder where I am at that point. Was I on the flight? Did I miss it? Why would I miss the last leg of a multipoint flight?
What if I am "hidden city ticketing" to another country? Isn't that going to get sticky with immigration? I mean, I fly to Spain and then Istanbul, and my airline continues on to Kazakhstan, but I'm hanging out at the Hagia Sophia. What are the Kazakhstani authorities gonna say about my visa plans or however I faked out everybody and made them believe I was going there? Isn't Turkey going to have something to say about my continued presence when I never should've left the airport?
Logistically, it must really mess up the airlines' planning and operations. If people get in the habit of reserving seats they don't intend to fart in, then that's a capacity problem. Passengers gonna get bumped. Imagine a whole 737 full of "hidden city ticketer" seats that doesn't even bother departing.
Yeah, I get it, passengers save money. Solve the fare imbalance somehow, airlines! Obviously if hackers are hacking you then you need a mitigation, and it's bad optics for you to detain and interrogate a script kiddie, such that he makes a headline on the Daily Fail. COVID-19 hurt us all; suck it up.