We do it because all of Catholicism is not under one head.
The EO do not adhere to the doctrines of Rome and by using the generic term of *Catholic* when addressing something specific to Rome, that would by default include the EO.
Since the error being addressed is specific to Rome and not the EO, it would be wrong to include the EO in something they don't agree with or adhere to in the first place.
As for the nonsense about them being in communion, the Great Schism happened in 1054 and the Roman Rite and the EO have been at odds since then with each side claiming that they are the original form of Catholicism as it was established by the apostles, and the other being the ones in schism.
And I actually see the EO holding to a more Scriptural view of Christianity than Rome does.
It kinda irks me when Roman Catholics assert that they alone have exclusive rights to call themselves the “Catholic Church” (they even capitalize Church) and they get huffy when anyone notes they don’t OWN the label. That word “catholic” was a Latin term that meant “universal”, so at one time it was understood that talking about the catholic church was referring to the whole of the Christian believers united in the true faith of Jesus Christ. According to some Roman Catholic teaching, only they can validly call themselves the Church! Other Christian assemblies are smugly called “ecclesial communities” and not churches! Such inanity when “ecclesia” (or assembly) was where the term “church” was derived.