As opposed to the northern/southern hemisphere that has:
- a clear reference point (equators)
- traditionally we don’t model a rotation north/south
Western/eastern hemisphere
- doesn’t have a clear reference (Greenwich?)
- traditionally we model rotation on the north/south axis so western/eastern would depend from the time
The prime meridian is a completely arbitrary line and the “western hemisphere” (defined in terms of it) is equally arbitrary. We need a prime meridian though, if only in order to draw the international date line on the opposite side of it.
The prime meridian roughly separates the two major clusters of land mass on the planet, which have undergone wildly different patterns of human settlement and development.
The line itself is arbitrary, but the concept of the hemispheres as shorthand geopolitical labels for these geographic clusters is very much not arbitrary.
If that were the goal it would be further west so that all of Europe and Africa would be east of it.