Simple answer.
1. The manufacturer of the integrated amp has not chosen to accommodate a sub as an option because his market analysis told him that it was not cost effective.
2. The manufacturer of the subwoofer has not chosen to accommodate speaker level input from an integrated amp as an option because his market analysis told him that it was not cost effective.
The HT market (which is the dominant market for subs) and the audiophile small integrated amp market have almost no overlap.
1. The manufacturer of the integrated amp has not chosen to accommodate a sub as an option because his market analysis told him that it was not cost effective.
2. The manufacturer of the subwoofer has not chosen to accommodate speaker level input from an integrated amp as an option because his market analysis told him that it was not cost effective.
The HT market (which is the dominant market for subs) and the audiophile small integrated amp market have almost no overlap.