Think of a press conference as a job interview. Politicians take a job interview every time they talk to anybody publicly. In job interviews, you do not want to be caught lying and you must always appear polite and respectful, but you aim to steer the conversation towards the points that you want to emphasize. The job of an interviewer isn’t to ask trick questions and to trap people in lies, but to try to steer the discussion onto substantive issues to see whether the candidate is suitable for the position. An interviewer who is rude or hostile is not acting in a professional manner. Interviewing is much more difficult than most people realize, and most people are actually very poor at doing it, people tend to hire their “reflection” – candidates who can portray themselves as holding the same values and opinions as the interviewer. This is why companies end up with so many useless employees.
So, politicians are basically professional job applicants with years of experience (and deliberate practice) at politely deflecting awkward questions and steering the discussion to their own talking points. They also have professional staff who research important issues and prepare stock answers for obvious questions. Any politician who doesn’t master this process is very quickly not a politician (Sarah Palin, for example). Interviewers are mostly much less skilled and less well prepared because they have to interview a wide range of different people and don’t have time and resources to exhaustively research for each interview (unless they’re a superstar working for a large news organization that has teams of researchers).
Politicians therefore give vague answers to questions because they know that’s how to keep their jobs, just like any intelligent job applicant knows to give the vaguest answer possible that isn’t an obvious dodge. It works, that’s what voters reward, that’s why politicians do it.
Donald Trump broke this process because he recognized that he had a solid core of supporters who responded positively to bad behaviour. It worked in the short term, but in the long term, it turned moderate voters against him and he will go down in the record books as the most unpopular and least successful president in modern history. Problem is, it took several years for journalists to adapt to his behaviour and learn how to deal with him.