Simple: anything to do with safety, flight instruments, engines, and fuselage is maintained and handled by an entirely different team of people, who have to get specifically certified to work on the parts that they work with, with internationally coordinated oversight and monitoring efforts. Being lax on these would mean the airline is deemed unsafe to fly, and hundreds, if not thousands, of people would lose their certifications/jobs/credibility.

Inside the aircraft, as long as the seats allow for evacuation within a stipulated time frame, have enough space in between them, and do not lead to weight distribution changing significantly, an airline can do whatever it wants without a aviation safety regulator stepping in.

This might be under the purview of a consumer rights regulator, though.