Parties work a bit differently in the US vs e.g. Israel.

In Israel, party insiders choose their politicians. If you want different candidates than an existing party is offering, you have to make your own new party with your own new list.

By contrast, in the US, parties run primary elections where voters pick the candidates. The specifics depend on the state, but in most states the election is held for registered members of that party.

Americans aren’t idiots. Most know third party candidates don’t do well in plurality elections. So smart progressives, alt-right etc. politicians don’t run as a third party candidate against mainstream Democrats and Republicans. Instead, they primary an incumbent Democrat or Republican, like Alexandria Ocasio Cortez, or join the primary when the incumbent retired like Marjorie Taylor Greene.

Somewhere like Israel, Alexandria Ocasio Cortez and Joe Manchin would be in two very different parties. In the US, they’re in the same party.

In places where RCV is passed, you absolutely see more candidates running and getting decent percentages of the vote. Because that isn’t a terrible strategy any more. Someone like AOC might have run as a Progressive or something rather than primarying the Democrat.