As the platter density increased, the head could glide over more data at the same spindle speed and you would get increasingly higher sequential speed. But you would not get much better latency - physically moving the head to a different track could not be done significantly faster. With each new generation, you would get only marginally better speed for many workloads that users actually care about. The latest HDDs could saturate SATA bus with sequential reads, but would still dip into KB territory (not MB, let alone GB) for sufficiently random-access workloads.
SSDs are similar in a sense that they can be massively parallelized for the increased throughput, but the latency of an individual cell is much harder to improve. Benchmarks will saturate the I/O queue and reach astronomical numbers by reading many cells in parallel, but for most desktop users, the queue depth of 1 (and the individual cell latency) is probably more relevant. That's why a lowly SATA SSD will be only marginally slower than the newest NVMe SSD for booting an OS or loading a game.