Interesting post. For most of Protestantism, the true church is invisible and therefore unity is already a reality. So what real unity is left to desire? The outward is not seen to be as real as the inward. They think that the “plain” meaning of scripture should lead everyone to their own system of belief, so each group proceeds to pile up proof texts to attract adherents, but when push comes to shove, most consider unity to be invisible in Christ.
Also, it would take ecclesial authority to bring outward unity and Protestantism is deeply critical of nearly all authority over the individual.
I’m with you, unity is important and not something separate from truth, as if we had to choose between the two.