Yeah but Windows was created for existing hardware, in fact at first even running on an existing OS. IBM PC compatibles were a thing since the 80s on which DOS flavour from various vendor were running which in turn ran on x86.
Microsoft actually waited really long until they combined normal end user Windows with NT which was it's own OS since early 90s but also had a lot of restrictions when it came to drivers, end user Apps, Games in particular.
Google bootstrapped it all at once, that's quite an effort. That said, some phones like those of OnePlus or FairPhone have really long support times.