I've spent a long time wondering the same thing. The standard answer is that it's fallout from the anti-anti-piracy cat and mouse game. The more conspiratorial answer is that bandwidth is expensive and streaming sites will take any excuse to serve you a lower resolution than what you actually paid for, while still being able to say that they technically support 4K.
There are sensible-ish technical reasons why they can't deliver DRM'd 4K on linux, but when browser extensions can upgrade you to 4K there are no excuses on the technical level.
The point is, people usually pay because it is more convenient for them than getting it illegally.
But when I have to fiddle around for 30 Minutes to see a picture (it worked before until it suddenly didn't work anymore), pirating the movie is suddenly the better option. Because I certainly don't see a point in paying and wasting more of my time.
And the piracy cat and mouse game is stupid, as in the End it's always Available illegaly, except for the people developing and selling DRM
There's a fine line, reduce quality too far and customer satisfaction drops. Some customers will have a higher tolerance to dropping quality than others, but you've got to draw the line somewhere.
So you do studies, you look at the impact of quality changes to customer churn and then you move the line appropriately.
Netflix is responsible for 15% of global internet traffic. That's expensive no matter how you slice it, and dropping that by a mere 1% is a huge saving.
There are sensible-ish technical reasons why they can't deliver DRM'd 4K on linux, but when browser extensions can upgrade you to 4K there are no excuses on the technical level.