Apple Watch Compass inaccurate, Why?

The Apple Watch Compass is inaccurate, even if you don't wear it with a magnetic band/strap. Why is it so complicated (for an innovative company as Apple) to produce a compass version which work properly f.e. based on GPS and not on magnetic north.

There are some ways to improve the working of the Compass by moving your arm and making the number 8, use a not magnetic band/strap.

You can also check the accuracy of the shown measure by the red shadow area (so called accuracy indicator). But all these actions are tricks around the margins.

My question here is, does the new Apple Watch Ultra has the same problem, or if not why do they not deliver the same compass on the new Apple Watch 8?