The House of Representatives in the US gets voted in the exact same way that Parliament in the UK gets voted. Yet there isn't a single third party seat in the House. The problem is something non-electoral, like the third parties are not trying hard enough, or they are being blocked somehow, if they don't have even one seat in the House.
I don't think third parties even really try to organize and do the boring work of proving themselves in local/state office.
They are just spoilers; you don't see the Libertarians or Greens saying "you know what, forget the presidency, obviously we aren't going to win--let's field candidates for like mayor, city council, DA, state legislature, etc. in really swingy/purple districts and show people what we can do"
Yep, the pure delusional thinking required to seriously run for the presidency when they don't have a single elected federal position anywhere. It's beyond absurd.
Yep, but it actually breaks the UK system to have more than two parties so the existence of two parties there isn't actually a very good alternative. There's a lot of seats in the most recent election where the Conservative party lost only because the even further right wing Reform party took so many votes.
If I were to guess why third parties don't make much of a dint it'll be because successful movements gradually get incorporated into one party or the other via the primary system. Once a party has drained away the core appeal the third party or outside movement will flounder.
