Pretty ironic, seeing as the mobile market was Google's way of starting over with forced sandboxing and higher security. Instead, Android has a horrible security reputation due to the complete lack of updates most people receive.
Of course it's only Google's fault. You could blame it all on Samsung and friends, but the reason we have this situation in the first place is because the UI for stock Android was absolutely atrocious, which caused a split with manufacturers going their own way. Google seems to be working on it though, with Treble and whatever the next version of that is called. Hopefully, Google will be updating Samsung/Huawei phones with security updates in the not so distant future. Now all we need is for Google to support their own damn phones for 5+ years like Apple does. My OG Pixel has a year of updates left and it's laughable. 3 years is a joke.
Yes, this might be a reason why they went their own way, but it's not a reason why they don't update their OS once Google released updates. I agree, I had similar experience with Nexus tablet. OTOH I still get updates for my iPhone 5S...