Why do our leaders not get trained in leadership? Put simply, and speaking as an individual only, it’s because this is a democracy. A cynic might call the electoral process a popularity contest and, to some degree, they would be right.
Some institutions require leadership skills - the army, for instance - so they teach it, nurture it and develop it. In theory at least, success as a military commander involves a lot of leadership. Similarly, a dictatorship can impose standards WRT education, experience and leadership on its cadres; democracies don’t work that way.
For a politician in a democracy, ‘success’ is defined by the ability to convince a majority of voters that you are the best choice - and then keeping them convinced. Don’t get me wrong, I firmly believe in democracy, but as Churchill noted, it is the worst system of government - after all the others. Its trick knee is that a charming buffoon with a sack of unrealistic but glittering spending promises is often easier to elect than a brilliant person who wants to take effective but unpleasant steps to make the country or city work better in the long run. Too often, charm, charisma and a good hairstyle count more than intelligence, leadership and competence. This cycle continues until the voters catch on to the fact that things are not running as well as they might and turf the incumbent in favour of the next charmer.
(There is actually a second problem, voter qualifications, which reinforces the first. We have taken the moral stand that one’s franchise hinges not on wisdom, education or experience, but solely on the accident of one’s place of birth. The vote of a semi-literate drunkard counts just as much as that of a Nobel laureate. That may be the most moral way of doing things, but it can also be like letting a pack of six-year-olds choose the the menu for everyone, for every meal.)
Once in a while, somebody very competent gets in, but that tends to be as much despite the system as because of it. All the same, nobody has ever come up wth a better option that works in practice.