My assumption is that Apple has a better contract with Qualcomm, being their biggest customer (for now, until they completely move over to their custom modems). Apple probably also has been abstracting the firmware from the start inside iOS, while Android didn't until project treble.

Samsung & Pixel are now offering 7 years of updates for flagships, so it would seem it's no longer a hardware/support limitation and purely a financial decision by other android manufacturers, and by Samsung for their non S-series of phones.

TL;DR OEMs are deliberately choosing to not support their devices, not due to any limitations anymore (thanks to project treble).