Basically because women would be held accountable. That is the main reason. Women have the ultimate trump card in these situations and they know it unfortunately. Go watch those shows where men fall to the floor because they assumed they had a child. If they ask for a DNA test “you don’t trust me we are done”. If they never ask for one they could be stuck raising another persons kid. If your suggestion was to “opt in” to the test like you said. How many single men would be leaving hospitals? It’s basically like signing a contract blind. It’s manipulative and sexiest to men. But since it hardly affects anyone it really doesn’t matter right?