I'd put money that automatically handling the affinity per CCX would give you a good chunk of performance boost, but that it would require a substantial amount

I'd put money that automatically handling the affinity per CCX would give you a good chunk of performance boost, but that it would require a substantial amount of work to the OS that wouldn't be completed before benchmarks and reviews came out. Given that Ryzen 3/5/7 performance has improved since they were released, but only a few of the reviews have been updated, it's likely that AMD wanted to avoid that and put in Game Mode as a stopgap.