Render Farms are actually way more efficient than buying a powerful system. Even the most powerful system can only process so many frames a minute... and when we're rendering on the order of thousands of frames, it because undeniably more cost efficient to farm the work to many slower computers.
Basic math portion. The fastest computers in the HLP Farm rendered a Lt. Ash frame at 1080p in about 10 minutes. Let's say a threadripper could knock out 1 frame every 2 minutes (which is honestly pretty generous, even for a threadripper). We need to render about 3000 frames. That's 30 frames an hour, for 100 hours. Add in the farm (the math gets complicated because the average times varied based on the computers involved) that could render, on average, about 30 frames an hour already and you see that all our "slower" computers combined already are at least as fast as a single threadripper... and we didn't have to spend near $1000 just for the processor. In fact, about half the computers in the farm are old junk computers with barely more than 6 cores combined.