Why do we measure eustatic sea-level from the center of earth?

Question

Earth is rotating so it has a ellipsoidal shape. The measurement will therefore be different from equator and pole. So why do we measure eustatic sea-level from the center of earth?

Answer 1

To have consistent measurements, the eustatic sea-level has to be measured from a location that does not change. The center of earth is the only place on earth that does not experience. Therefore, this location was chosen.

Answer 2

I'm not qualified in Geophysics, but I can give a general answer. Measurements are often stated with reference to some parameter that is "global" , as opposed to "local". A global reference point will tend to average out local variations in the relevant parameters.

Sometimes the chosen reference seems arbitrary, but there is usually some kind of "mathematical convenience" that is part of the logic.

This seems to be a pretty good writeup on the specific question:

https://en.wikipedia.org/wiki/Eustatic_sea_level