The issue is the same as with statutory laws.

Some people, we as a society have agreed, cannot make that decision yet.

Incest in particular is difficult because no one but the state has an incentive/power to intervene.

Especially in familiar context where all the incentive is to "handle it within the family" when there's a power imbalance and if there isn't the incentive is to hide that it happened to avoid dealing with the consequences.

Victims in that case must convince a parent or sibling to stop and convince the rest it wasn't ok. Which, even beyond familial pressure, is just hard to do... So we accept the states intervention, to supersede the local power dynamics and intervene directly.

Same issue is with abuse victims, people can't always find their own justice, and in that world sometimes we as a society must pursue it for them.