PCs are generally based around the X86 chip architecture which is an open standard. PCs are basically modular and lots of manufacturers make components that are interchangeable, creating a huge variety of possible hardware. Hardware suppliers also sell.to big companies and individuals. It’s therefore in their interest to distribute their drivers freely even if closed source. If hardware breaks it can be replaced and the PC keeps going.

Mobile devices are closed standards. They use a more limited range of off the shelf components which are deeply integrated into a device, and the hardware suppliers provide their drivers to the device manufacturer or the device manufacturer builds their own drivers and custom version of the os. Hardware can have very long retail lives selling for years and still being functional, so the manufacturers have an incentive to keep drivers available and even update them.

It means mobile devices are more locked down, and the hardware drivers harder to come by. This makes it hard to build custom OS for them and therefore when the device comes to the end of its support from the maker there is limited options to keep it running securely.

It’s effectively a type of planned obscelence that keeps the mobile industry going. Manufacturers stop supporting old devices (because it provides no income) and then consumers have to buy new ones as no one can provide the security patches to keep them secure.

So for mobile there is nothing to force Android or IOS to be kept up to date for old devices. The money is in new devices, and for Android manufacturers are responsible for the mobile device anyway. While for PC it’s in Microsofts interests to keep updating and keeping devices secure via Windows becuase devices have long lifespans and old components can be in the PC ecosystem for decades. Similarly Linux is able to support hardware for a long time because drivers are more freely available and long lifespans to hardware incentivise people to put the effort in to write open drivers when they’re not there.