Apple only uses Qualcomm chips as modems. Almost everyone else uses Qualcomm chips as main SoCs.

Now, could hardware vendors tell Qualcomm to go pound sand and run their own support for old SoCs? Yes they could. Do they want to? Hell no, supporting old devices doesn't make any money.

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).