I am not able to answer with a proper, argumented answer, so feel free to downvote me into oblivion if I am wrong (but please correct me so we can learn something new). I think that the reason is that relational databases are centered on the relational model, which in turn is based on something I know nothing about called "first order logic". What you may ask probably does not conceptually fit in the mathematical/logical framework relational databases are built upon. Moreover, what you ask is generally solved easily by graph databases, giving more hints that it's the underlying conceptualization of the database that conflicts with what you want to achieve.