Although overprovisioning is part of extending the drive's lifetime, its main use today is to improve performance by using that space as faster SLC memory (which also wears more slowly). SSD controllers have very sophisticated algorithms to reduce wear on the cells as much as possible.
Provisioning of space for an extended lifetime is a statistical problem. If all cells in a drive take the exact same amount of time (or write cycles) to go bad and all are used equally, it's pointless to reserve any number of cells for replacement of bad cells, as you would just end up writing to the same cells more often. However, each cell is slightly different and not all of them are used to the same degree. Cells that die unusually quickly may be replaced, but by the time that a drive needs 10% of its total capacity to replace bad cells, it's already dying fast, because good wear-leveling algorithms make sure that all cells die more or less at the same time.