Because it is not just a matter of opinion or pride or arrogance or stubborness. Both sides believe without any doubt that their formulations are consistent and correct. Considering that the Church has been linguistically an apartheid since the 5th century, such diverity of formulations is a product of a long path of linguistic, cultural and historical events that separated them.
If anything, the degree of similitude, once the shell is cracked is much more telling of one Church than of two. For one, most importantly, both have apostolic authority, valid clargy and sacraments. Neither one can deny the other any more than they can deny each other of being Christian.
Okay, so the shell doesn't matter, and we can dispense with any further considerations of the shell. The core is the essential matter at hand. The core is the same in the Roman Catholic and Orthodox Churches. So what is holding up reunion?