Liberals often call Leftists excluding liberals from the Left "purity testing," as though the difference is merely in quantitative degrees, rather than qualitative. If the difference between Leftists and libetals is indeed merely quantitative, wanting the same thing but in greater or lesser extents, then the Liberals would be correct, however opposition to Capitalism itself and support for Socialism fundamentally represents a qualitative shift.
For Leftists, Social Democracy, or Welfare Capitalism, isn't actually a solution. The countries seen as "success stories" like the Nordics rely on Imperialism, they aren't closed loop economies. Further, their conditions are deteriorating as wealth concentrates. Leftists therefore aren't letting "perfect" be the enemy of "good," it's that Liberalism is built on a brutal system of international plunder, and is on a death spiral as liberal countries increasingly pivot more to the right. Climate Change is still an existential threat. Liberalism isn't a solution.
That's why there's friction between progressive liberals and Leftists.