The main issue is that OEMs make too many device models with unnecessary variations for carriers/regions and make too many changes to AOSP. It's extremely hard for them to properly maintain all of it.

Qualcomm offers up to 8 years of updates from platform launch. Getting around 7 years of updates requires OEMs to use the latest and great platform combined with paying Qualcomm a lot of money for long term support. It may cost a million dollars or more for each year of support. OEMs also need similar support for other components but that mostly means choosing decent components.

Providing proper updates has a cost most OEMs haven't been willing to pay. Pixels and Samsung flagships have been the exceptions. Samsung doesn't properly update most devices, only flagships, and it's still worse than Pixels in important ways. Samsung has also been closest to having all the hardware-based security features we need but doesn't let us use a lot of those due to crippling devices if they're ever unlocked.

Our partnership with Motorola Mobility partly involves them improving their devices to meet all of our requirements which was already largely happening. It also requires porting GrapheneOS to their devices and fully supporting Snapdragon again including having hardware memory tagging support on it for the first time. No one is currently using hardware memory tagging in production on Snapdragon let alone for the entire kernel and userspace as we do so it's going to be a lot of work. Motorola is going to be helping with all of this. They're also going to provide us more minimal hardware support code without unnecessary changes not needed for AOSP / GrapheneOS. A bunch of GrapheneOS features need to be ported and the device support code needs to be made compatible with our changes too including but not limited to fixing memory corruption bugs.