Well, for starters, they didn't (I can't believe I'm saying this again) bury themselves in debt trying to attain Netflix or Hulu-like scale.
Sundance has a mixture of content from small filmmakers, AMC's own library and, it appears some off-network shows. None of that costs a lot, and they don't have a lot of it. Even if it loses a little money, it's probably fine within the context of AMC's overall balance sheet.
Similarly, Starz has a limited library, much of it original content made to a budget, stuff from the Lionsgate film library and, again some off-network stuff (Laramie? Wagon Train? The Rifleman?). And, like Sundance, if it loses some money, it's probably a wash once it's part of the overall Lionsgate budget.
You DO understand that not every streamer has the same business model, the same costs or the same level of indebtedness, right?