I wouldn't say AMD is closing the gap in CPUs. They're ahead of Intel and are widening that lead.
For whatever reason, Nvidia hasn't stagnated the way Intel has. Nvidia has continued rapid progress in GPUs even though they have a dominant position (and a near monopoly in the datacenter).
Perhaps because they don't have as much of a lead as Intel used to? Intel was a monopoly, plain and simple, but AMD has always been a contender when it comes to GPUs.