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.