It could be that Samsung pays for the long time support. In that case the drivers are based on a support license. But Fairphone did say they are working on an agreement to have longer vendor support from Qualcomm. So maybe they are trying to get the same deal.
It could also be a marketing thing from Samsung. Releasing an Android 15 update and soon after that stop their support. My Pixel 3 got Android 12 and 2-3 months later the phone got its last update.
It’s also not just about the drivers, it’s also about getting the vendor security updates. Those are on a monthly basis. So FP isn’t saved by just getting the drivers.