The short answer is that because they don't want to deal with the shame they have internalized from their childhood or any of the bad deeds they have done, then they must project that onto you.

This is because to a narcissist someone can only be all good or all bad, and so a narcissist cannot take responsibility for a single bad thing that they have done, otherwise they are all bad.

This means that they must project it onto you or someone else, but it can never ever be their fault.

If you find a way to make something their fault so that it sticks, they experience a narcissistic injury or collapse.

If you find it helpful, you can go here for the long answer: https://medium.com/mr-plan-publication/why-narcissists-hate-you-but-cant-let-you-go-revealed-eec2c65679a1