This works because almost all the US uses first-past-the-post elections for the Presidential general election. So you get outcomes like this:
Scenario 1:
Biden: 10 votes
Trump: 9 votes
Kennedy/Stein/West: 0 votes
Biden wins the state
Scenario 2:
Biden: 9 votes
Trump: 9 votes
Kennedy/Stein/West: 1 vote
Tied vote, decided by game of chance/lawsuit
Scenario 3:
Biden: 8 votes
Trump: 9 votes
Kennedy/Stein/West: 2 votes
Trump wins the state
This is why you see huge financial support from Republican billionaires for third party candidates who have no chance of winning.
You’re using a very narrow definition of coalition — the Democratic party is a coalition of groups within the US.
Despite what you say, there are very real differences between the parties, and ones which make the Democrats a group where it is possible, at least some of the time, to get good policy. It’s happened before, and it will happen again if we give them the power to act.