Because you can't get a message out to all 300million people normally, let alone when there are billions of dollars of propaganda working against you is very difficult. Because even if a US 3rd parties were to gain significant power they don't have the institutional knowledge to even keep the lights on, whichever opposition would eat them alive. Because if they did somehow manage to get momentum and take over they would immediately develop the exact same problems of previous political parties because these are systemic problems with your political system.

Why not invest in a strategy that doesn't makes things even worse when you fail. Political reform that would allow 3+ parties to exist are just as big of a long shot as getting a third party elected, but it doesn't split the vote and actually fixes the problem long term.

But hey I'm not an American, I have 5 viable parties to choose from. so you do you.