I don't see a problem. They are the Republican and Democratic party candidates. The members of those parties can nominate whomever they want. There's nothing in the constitution that says anything about how political parties chose their candidates. Primaries didn't always exist https://en.wikipedia.org/wiki/United_States_presidential_pri...
I don't understand how business monopolies are considered bad, but political monopolies, which strongly affect the lives of over 300 million people who have little choice, are OK.