Because of primaries.
As the USA allows anyone to run for President, and the two big parties throw the choice of their nomination open to voters through primaries, you end up with a long primary election campaign that only the richest candidates can afford. Especially as the states don't all hold primaries on the same day and it's spread over several months. So who you get is partly determined by wealth, and endurance to fight a campaign over a year long. And when you consider what gets done to them over that campaign time, I can only conclude that you'd have to be a masochist or someone with no feelings to WANT to be President. No normal person would.
In other countries, they don't do this. Each party itself chooses its leader as and when it needs to choose one, using whatever election process it decides to have amongst its actual members, so it already has a candidate for the national leader and there just needs to be the actual general election. The whole thing is over much quicker, and each party will undoubtedly have chosen someone with at least some experience of politics.
In a parliamentary system, the choice of candidates is restricted even further. The country is headed by a figurehead president or a monarch, and the head of government is the Prime Minister. The Prime Minister is expected to be, or is even required to be, a member of the parliament, so each party (at least if it has a chance of winning an election and forming the government) will only elect a leader from amongst their members who have already been elected to the parliament. It'll be someone the other party members in the parliament can work with, and who probably has some experience of not only being in the parliament, but also of being a minister in the government.
To take the UK as an example of this because I know it best, the Conservative Party chooses its leader by allowing about a week for nominations by its MPs from amongst themselves, then having the MPs vote by exhaustive ballot and knocking out the candidate with least votes each time. When they're down to two, it gets thrown to a postal ballot of all party members. Last time, when they got down to two, one candidate dropped out so they never got to the postal ballot stage, and had a new leader in about two weeks. Labour does much the same thing except it doesn't have the MPs narrow it down to two, it goes straight to a postal ballot of all candidates.
Having done this, there's always a candidate for Prime Minister already in place, and it's possible to have a snap general election. Consider what happened in 2016 - Theresa May decided to have one in the hope of getting an increased majority in the House of Commons, the Commons voted by the required 2/3 majority to have one, the election campaign lasted just over a month and then there was the election. All done before anyone has time to get bored with election politics. In fact it is required by law that once Parliament is dissolved, the general election must be held 25 working days later. There are also legal limits on campaign spending by individuals and that helps ensure the rich don't have an advantage.
Under that system, neither Trump nor Clinton would have been candidates because neither of them were in the House of Representatives. Is this sounding better?