This is not entirely true. Democracy is a broad term and different democracies work very differently.
In Germany for example political parties are quite powerful, politicians have to be elected by party members, there are no primaries like in the US. Being a politician in parliament or becoming chancellor in Germany requires a political career that takes quite some time.
This excludes people in ways that are both good and bad. On the good side someone like Trump would have no chance at all because he lacks the political track record and relationships. On the bad side politicians in Germany are not a very diverse group. There is a strong bias towards older people, people who've been in politics for their entire life and people in professions that allow for spending time as a politician in parliament for a couple of years and continuing in their regular job afterwards. In practice this means teachers and lawyers, at least in Germany.