Because that's not how it works. First past the post means there's only ever 2 parties.
You want to change that, cool, but you have to change it first before you can realistically vote 3rd party in a presidential election.
And since presidential elections are determined state by state, you have to change the first past the post system state by state. Maine and Nebraska (iirc) have proportional representatives, so that's 2 down, 48 to go.
Since the state will never change that alone, people should organize to vote for third party themselves.
If the poll says that most people will vote for a specific third party if they come up as a majority, then there is no risk in throwing your vote away.
My point is, I guess, organize elections without support from the state. The elections where you show who you would really vote for or the law or the system you would really vote for, without any obligations. Then when peoples choice is clearly seen, you can have courage to actually vote for what the most people believe in.
Yes, nobody is advocating for third party to be always in power, since obviously then they are not a third party anymore. It just like no one is advocating for opposition to the government to always be in power, because that makes no sense, but the change of government and voting for that opposition does make sense.
It would be better for people to choose moderates then these people we have now. I am sure majority of people don't support genocide in Gaza. But if people don't understand that voting for third party is necessary, we should work on that, we shouldn't give up.