This is because of the European Court of Human Rights that prevents courts from handing hard life sentences. There must be a point when prisoners have to become eligible for parole and have the possibility of being released.

The UK has had issues about this and has had to find a way to adapt: Whole-life ordesr have to be reviewed after some time in order to give a chance of release (at least on paper).

For the same reason, e.g. in France life sentences come with a maximum of 30 years before becoming eligible for parole. What they do is then that they simply keep denying parole for prisoners they want to keep inside.