Why does Beingness desire Earth experience from Ego?

[anagogy]

Surely the Ego could be constricted even more into a much denser dimension. Which does exist. We just chose this level??

The much denser (in the sense of mental freedom) dimension you are referring to, from my perspective, is 1st density. There are many varieties of ego, or self, and yellow ray identification we are familiar with as ego isn't the beginning. So the answer is, it isn't just choosing this level -- it explores the entire continuum between the form and formlessness. Absolute constriction, and absolute unboundedness.