Zero days aren't the big driver of botnets, there are millions (if not hundreds of millions) of very cheap, very old, android devices out there. If you look at the periodic stats Google releases >50% of devices are running an Android version <= 10. Something like 20% of Android devices (at least according to the stats Google provides) running Android <= 5.

Per earlier this year: https://m.gsmarena.com/android_13_is_now_running_on_12_of_devices_in_the_wild-news-58244.php

I'm assuming these stats don't even cover a huge number of cheap Indian or Chinese devices too, those don't come with Google services at all.