This isn't the same thing whatsoever. This frequently involves reverse engineering, see Asahi Linux for an example. That also doesn't mean that things work well

This isn't the same thing whatsoever. This frequently involves reverse engineering, see Asahi Linux for an example. That also doesn't mean that things work well or in their entirety, e.g., dock connectors on ThinkPads might not work. Google is a corp that is bound by the laws of the USA. If they were to reverse engineer a SoC by Qualcomm, they may infringe on their intellectual property rights amidst other issues. Not to mention, it's much easier to make their own chip, which they have done. Third-parties sometimes write alternatives like e/OS and LineageOS, but the devices they're written for are seldom functionally complete, just like Linux isn't in terms of hardware support a lot of the time.