Because they're not. It's virtually impossible to start a meaningful new party in the US due to the FPTP system, so you are stuck with whoever the two legacy parties decide to nominate according to their own rules.
Compare Germany: nine parties represented in the federal parliament, a proportional system ensuring that getting 50%+1 of the vote doesn't mean you get 100% of the power, and relative ease of splitting and fusing parties making it so that previously unrepresented political views can easily gain representation (e.g. the socially conservative Russophilic left-wing party "BSW" recently splitting from the standard left-wing party).
> Who else beats Trump?
Most people selected out of the telephone directory at random could have beaten Trump. No, this probably doesn't include Pete Buttigieg.
> The fact is America would be happy with no one. But we got who America wanted
These two sentences contradict each other.
I just see no appetite for a 3rd party, much less nine in the US. It was amazing how people would complain that Harris provided no details about her plans, when 15 minutes on her website provided more detail than most people would care for (although certainly not at the level of detail any wonk would want). Do you think people are really going investigate nine candidates?
> Most people selected out of the telephone directory at random could have beaten Trump. No, this probably doesn't include Pete Buttigieg.
Given that every Republican can't seem to beat him there must be some odd bias in the phone books you have.
> These two sentences contradict each other.
They don't. We got who we wanted -- we just aren't happy with it. And wouldn't be happy with anyone. No contradiction.