Because having one OS for a device with parental protections that parents can install is enough to achieve the goal, so the laws are obviously overreaching by mandating age controls for every OS when that's clearly not necessary. Having one Linux with age control that parents can install is much less intrusive and much more achievable than mandating every minuscule Linux distribution developed by hobbyists in their spare time to implement age control (which is practically impossible and never going to happen). And let's not even get started on the Internet of things...