If a doctor has to defend themselves in court they have to pay a lawyer, spend a bunch of time on it and it's a huge stressor. Much easier to just stay quiet. Almost all doctors would step in and save a live if there was something specific that they needed to be done. But in an airplane, there's barely any equipment - all of the stewards are going to be trained in first aid and cpr, so are likely better able to manage anything that can be done. The only reason they ask for a doctor is because it's a trope that people expect (so people would be upset if they didn't, and so didn't appear to be doing all they could), and to share the responsibility.