The reason for this is that CPU upgrades on the same board were/are very viable on SP3.
Doing that on Intel platforms just wasn't done for basically ever, it was never worth it. But upgrade to Milan from Naples or Rome is very appealing.
So SP3 CPUs are much more common used than the boards, simply because more of them were made. This is probably very bad for hobbyists, the boards are not going to get cheap until the entire platform is obsolete.
Doing that on Intel platforms just wasn't done for basically ever, it was never worth it. But upgrade to Milan from Naples or Rome is very appealing.
So SP3 CPUs are much more common used than the boards, simply because more of them were made. This is probably very bad for hobbyists, the boards are not going to get cheap until the entire platform is obsolete.