Comment by Nevermark

In the US, there are tremendous structural barriers for third parties. They exist, it is just extremely difficult for them.

The centralization of power of each of the two dominant parties nationally at the expense of a more decentralized parties with strong state variability as in the past, makes it even more difficult for third parties to gain traction against all that coordination.

Perot had the best chance, but managed to blow it by bowing out and then back in.

I do think you are right, that times of great dissatisfaction are rare openings for third party candidates, if someone special enough appears. 2020 would have been a great election for that - but an inspiring third party candidate can't be manufactured on demand.