Netflix is in fact working very hard to optimize the quality per bandwidth at low bitrates. Megha Manohara from Netflix had a conference talk about it last year:
https://www.youtube.com/watch?v=S9E9xUbcYAM
Their goal is to have video of acceptable quality at 250 kbit/s, which makes Netflix an option for a lot more people worldwide.
That's part of it, but I think you'd be astonished at how much IP transit can end up costing when you're distributing high-quality video. It's pretty staggering how much bandwidth can cost. I would not be surprised in the slightest if at least 1/4 of Netflix's total engineering spend is on network transit, so any savings that are per-video can have a huge impact.
Source: I work at a company that distributes a lot of high-quality video :)
I agree with your overall point though, that at some point the extra cost for encode isn't worth it.