Ok. Where's the list of reasons? And please confine it to items relevant to the issue at hand. Don't clutter it with references to situations where someone has acted as a father to a child for a significant period of time.
Part of the reason, has been the ABA fighting with extreme vigilance to oppose use of DNA tests to eliminate somone as the father. This viligance is strong that involves not just the local california bar, but also from the national level, with and including threats against democrats to withold campaign donations. (this is a big deal for them, to put it nicely, they don't fight for causes with this kind of use of force normally).
To soft sell this to the public, they have slanted it. Now it becomes, what if somone, knows that this is not there child and yet has raised them for 15 years, and decides to up and leave, then what? or What if the mans wife cheated, had a child, but the husband forgave her, he could use the child not being his as leverage till the child is 18. What if the mother is the victim of a rape, and 5 years later there is a divorce?
If you notice, its alot of populism slants, and mention of the real father is always very limited or non-existant. The reality is, is that the real father should pay, and also be obligated to back pay child support since they weren't there in the first place. In other cases, if the man knows this isn't his child, he should then either acknowledge or not what his future obligations should be.