OTA updates on Android phones means downloading (often 250mb) files -> asking the user to reboot to recovery -> installer takes a few minutes -> boots back into the phone.
Most people keep their phone on 24-7. So this is not as simple as updating software apps in the background w/o reboot.
edit - Also, Microsoft has complicated relationships with many large vendors and probably tests on a LOT of permutations for each patch vs whatever Google does for Android. I am surprised to find myself sympathizing with Microsoft.... that being said, this is digressing. MSFT should have a universal fast track for 0days and decouple it from the standard upgrade process. 90 days should be enough, and have no real problem with Google's actions.
Additionally, Windows OS has a long history, is much more stable, and Microsoft controls the majority of the codebase unlike Google.
Android's codebase is built on top of quite a rickety legacy foundation of old patchworked linux kernels that have to be in sync with drivers from proprietary manufacturers like Qualcomm - although is has improved a lot in recent years.
Yet, I do agree Google/Android vendors should be pushing updates at a much higher frequency.
Non-Nexus devices distributed with OS modified by 3rd-party vendors and/or cheap/low-powered Androids are the big reason why Android phones stats show a lack of updates. It is not because of Google - which does push out OS updates in a timely way.
Yet Google still releases patch/delta very infrequently which is what we're comparing vs Windows.