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.
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.