It’s because we’re at the limits of the human visual system. The difference in pixel pitch between 4k and 8k at the distances we watch TV is literally imperceptible.

It also doesn’t help that there’s not much content authored and distributed for higher resolutions. It’s exponentially more expensive to produce, store, and deliver.

Home Internet connections on average aren’t any better than they were ten years ago, either, at least not in the US. I doubt a lot of them can even support 8k streaming, let alone with anyone else using it at the same time.