The reason is that liberal democracies are dictatorships of capital, where the elites have massive power over who even gets to run as a candidate, what ideas are acceptable and so on. We can't win in this system because the system is rigged from beginning to end, and even when we do miraculously win we can get our elected officials killed and replaced. Look at what they did to Salvador Allende in Chile.

The game is rigged in such a way where one party can simply be extremely exploitative and violent and the other party can be 99% the same and sometimes win by saying "we're not THOSE guys". You need money for campaigns, so you can't realistically threaten billionaires, and if you do betray them they won't have any qualms getting rid of you illegally.

It's not our collective fault, or purity testing, or whatever else, it's the system working as intended for the people who pull the strings, just look at the people in the Epstein files and notice that both parties engaged in the debauchery, look at how Bernie was betrayed by the DNC, and so on and on.

But hey, if purity testing is not being okay with genocide or invading and bombing people in the global south I'm glad there are leftists who do purity testing.