Because in reality, majority who complained probably don't have iPhones anyway. It's the internet echo chamber talking. It's worse when Apple's explanations are twisted by the tech bloggers as "intentionally slowing down."
A lot of the complaints are on the 6S, which Apple had run a free battery replacement program since 2016. Funny no tech bloggers/news media even mentioned that.
As for other manufacturers, it's most likely because nobody use a typical Android phones that long. And even if they did, most recent Android phones have beefy enough battery (at least 3000mAh and up, even budget ones), that it would've taken longer for any battery degradation to be felt.
At least Apple offers replacement. I had an HTC One M7, and its battery expanded, opening up the screen/phone. HTC Singapore refused any support/replacement, even if I wanted to pay for it.