We agreed among ourselves centuries ago to communicate in a language not burdened with fourth declensions and such, so why are we occasionally fixated with these in Modern English?

One of the funny things about language is that though it is rules-based and conventional, those rules and conventions are not something consciously agreed to by its speakers. We tend to view our language's grammar as something natural and inherent, while other languages are burdened with outré and louche grammatical gimmicks. I have often wonder, in my spare moments, why the Slavic languages (save for Bulgarian) did not get rid of their cases, while all the Romance languages did. Also, English got rid of its case system, but German did not.

Ceci n'est pas un seing.