No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and Mammon – Matthew 6:24, KJV
In representative liberal democracies, educational/professional qualifications are not required of candidates to public office. Take an aspirant to the Prime Ministership or the Presidency. There are neither schools offering “Prime Minister 101” courses nor licensing requirements, as there are for would-be plumbers or lawyers.
Sometimes candidates may have somewhat related experience: say, a former state Premier could wind up in the top job. But generally that experience is neither fully compatible (say, a Premier must prioritise public health, a PM focuses on the economy) nor, as far as I can tell, is that the usual situation (how many premiers do you know went on to become PMs?).
In countries under a parliamentary system, unlike those under a presidential system, that extends to Cabinet members: health ministers don’t need to be doctors, say.
That’s why the public service is important: public servants are supposed to provide the expertise their political masters lack. (No doubt readers have heard about the “checks and balances” included in the institutional design of liberal democracies to prevent dysfunction. Arguably, the bureaucracy is another such check and balance.)